fix(nix): install shell completions with the flake package

[clay-good]

Status: Ready for review.

Closes #1740.

What was wrong

The Nix flake built and installed the openspec binary, but installed no shell
completions. openspec completion generate SHELL printed a working script, and nothing
in the package ever put one on disk:

$ nix build github:Fission-AI/OpenSpec
$ result/bin/openspec completion generate zsh   # prints a script
$ ls result/share
ls: cannot access 'result/share': No such file or directory

So a Nix user had completions available in principle and absent in practice. The
documented escape hatch, openspec completion install, writes into $HOME — the wrong
answer for a package manager whose whole contract is that the package carries its own
files.

This is the packaging half of the completion generator added after #242, not a request
for completion generation itself.

How it was fixed

postInstall runs the generator that already exists and hands the three scripts to
installShellCompletion, which places them at the standard Nix locations:

Shell	Path in the package
Bash	share/bash-completion/completions/openspec.bash
Fish	share/fish/vendor_completions.d/openspec.fish
Zsh	share/zsh/site-functions/_openspec

Three details worth naming:

• The generator is safe to run in the build sandbox. completion generate renders a
  static command registry — no project, no filesystem discovery. OPENSPEC_TELEMETRY=0
  keeps it off the network (the same flag short-circuits the update check, per
  src/core/version-check.ts:34-36), so the build stays hermetic even where the sandbox
  itself would not enforce that.
• Generation failures fail the build. The scripts are written to real files first
  rather than passed through <(...) process substitution, whose exit status bash
  discards — a broken generator would otherwise install an empty completion silently.
• The canExecute guard is hygiene, not a live branch. pkgs = nixpkgs.legacyPackages.${system} is the native package set for each supported system,
  so buildPlatform == hostPlatform and the guard is true for every package this flake
  exposes (verified below). It stays because it is the nixpkgs convention and matters if
  the derivation is reused in a real cross context via an overlay — but no user of this
  flake can end up with the completions silently missing.

Two CI changes back it up. The existing "Verify build output" step now asserts the three
files exist, are non-empty, and that the Zsh script still opens with #compdef openspec —
the line that makes it autoloadable. And the Nix job's path filter now also fires on
src/commands/completion.ts and src/core/completions/**: the Nix build runs the
generator now, so a change there can break packaging without touching flake.nix.
Without both, this regression stays invisible to CI, which is how it survived.

Proof it works

Built the flake from this branch in a clean nixos/nix container (aarch64-linux). Every
line below is observed output, not inference.

The files land where the shells look, and the shells actually use them:

=== nix profile add .#default ===
~/.nix-profile/share/bash-completion/completions/openspec.bash -> /nix/store/...-openspec-1.12.0/...
~/.nix-profile/share/fish/vendor_completions.d/openspec.fish   -> /nix/store/...-openspec-1.12.0/...
~/.nix-profile/share/zsh/site-functions/_openspec              -> /nix/store/...-openspec-1.12.0/...

=== functional completion ===
bash   $ _openspec_completion  with COMP_WORDS=(openspec val)   ->  validate
fish   $ complete -C 'openspec va'                              ->  validate  Validate changes and specs
fish   $ complete -c openspec | count                           ->  556
zsh    $ compinit; print $_comps[openspec]                      ->  _openspec

They sit in the profile next to nix's and git's own completions — the ordinary path a
user's shell already searches.

Everything else that was checked:

Check	Result
nix flake check	passes
x86_64-linux package's postInstall evaluated from aarch64	non-empty — the canExecute guard never omits completions here
Installed files vs. freshly generated	identical, all three
Generator run twice	byte-identical output (no timestamp or version stamped in)
Completion files across two full builds	identical, all three
$out/share contents	exactly the three files, nothing stray
bash -n / zsh -n / fish --no-execute	clean

On main, nothing in the derivation writes to $out/share, which matches the empty
result/share in the report.

Notes / nits

• nix build --rebuild reports the derivation "may not be deterministic" — that
  predates this PR. I checked rather than assumed: building main's flake, which has no
  postInstall at all, reports the same. With --keep-failed the single differing path is
  lib/node_modules/@fission-ai/openspec/node_modules/.bin/changeset, a dev-dependency
  binstub that dontNpmPrune = true keeps in the output. The three completion files are
  identical across builds. Worth its own issue; out of scope here.
• Follow-up this PR creates: the CLI's first-run tip still says
  Run 'openspec completion install' for a Nix user who now already has completions,
  because isInstalled() checks only $HOME paths (zsh-installer.ts:454, and the same
  in the bash/fish installers). Taking that advice writes a home copy that goes stale on
  the next nix profile upgrade, and whichever comes first in fpath wins. The general
  fix — retire the tip when completions are found anywhere the shell searches, e.g. under
  XDG_DATA_DIRS — is a runtime behavior change on a shared code path, so it does not
  belong in a packaging PR. Happy to file it.
• No changeset. Nothing in the npm package changes — this is flake-only plus docs and
  CI. Recent flake-only commits (#1633, #1718, #1745) followed the same convention.
  Happy to add one if you'd rather it show up in the release notes.
• pnpmDeps hash is untouched. The lockfile is unchanged, so no FOD rehash is needed,
  and scripts/update-flake.sh only rewrites that hash, so it will not clobber
  postInstall.
• PowerShell is deliberately left out: the generator supports it, but Nix has no standard
  PowerShell completion directory to install into.
• The new CI assertion sits inside the existing ci-nix-validation requirement
  ("the build output SHALL contain the openspec binary"). I left openspec/specs/
  untouched rather than editing a spec outside a change proposal — say the word and I'll
  open one adding a "completion scripts installed" scenario.
• Adjacent but separate: feat(cli): ship an offline man page #1784 adds a man page and leaves the flake alone; the two do not
  conflict.

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📝 Walkthrough

Walkthrough

The Nix package now generates and installs Bash, Fish, and Zsh completion scripts. CI validates their presence and Zsh autoload header. Documentation describes the packaged completion scripts and their standard locations.

Changes

Nix shell completions

Layer / File(s)	Summary
Generate and install shell completions flake.nix	The Nix derivation adds installShellFiles. Its postInstall phase generates Bash, Fish, and Zsh completions with telemetry disabled and installs them when the build platform can execute the host platform.
Validate and document packaged completions .github/workflows/ci.yml, docs/cli.md, docs/installation.md	CI runs for completion source changes and verifies that all three completion files are non-empty. It also verifies that the Zsh file starts with #compdef openspec. Documentation describes the packaged completion scripts and their standard locations.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🔵 Low · up to 7fc66

The Nix package adds packaged Bash, Fish, and Zsh completions with CI output checks, but documentation may imply those files are available in cross-built packages where generation is skipped. This is a bounded documentation risk to resolve before relying on that guidance.

Sequence Diagram(s)

sequenceDiagram
  participant NixBuild
  participant OpenSpec
  participant ShellInstaller
  NixBuild->>OpenSpec: Generate Bash, Fish, and Zsh completions
  OpenSpec-->>NixBuild: Return completion scripts
  NixBuild->>ShellInstaller: Install scripts at standard Nix paths

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address issue [#1740] by generating and installing Bash, Fish, and Zsh completion scripts through the Nix package integration. CI verifies that the scripts are present, non-empty, and corr…
Out of Scope Changes check	✅ Passed	All changes support the linked issue [#1740]. The CI updates, documentation updates, cross-compilation handling, and telemetry disabling directly support reliable Nix completion packaging.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: installing shell completions with the Nix flake package.

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• Create PR with unit tests
• Commit unit tests in branch fix/nix-flake-shell-completions

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Actionable comments posted: 1

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Inline comments:
In `@docs/cli.md`:
- Around line 1305-1306: Qualify the Nix completion documentation in docs/cli.md
lines 1305-1306 and docs/installation.md lines 158-162 to state that Bash, Fish,
and Zsh completions are installed only for execution-capable builds;
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