Follow-up to #266: make the pull request template actually reach GitHub

[schpetbot]

Follow-up to #266. Draft until #266 lands — this branch currently contains @maparent's commit too, and will be rebased down to just the delta once #266 is merged.

#266 adds -T/--template and a pr_template config option. The surface is right and is kept exactly as designed — same flag letter, same config key, same precedence. The mechanism can't work, though, and I couldn't find a variant of it that does.

The feature currently fails on every invocation

issue pr already passes --body <issue url>, and #266 appends --template beside it. gh refuses that pair:

$ linear issue pr CLI-113 -T tmpl.md
`--template` is not supported when using `--body` or `--body-file`
✗ Failed to create pull request

Worse, pr_template in config applies unconditionally, so setting it breaks issue pr entirely rather than only when the flag is passed.

Why the obvious repair is worse

Dropping --body to make room for --template looks right, but gh only consults a template when it's running interactively. Without a body, a non-TTY caller gets:

must provide `--title` and `--body` (or `--fill` ...) when not running interactively

…and no pull request at all. That trades a broken flag for a command broken in CI, scripts, and agents. Handing gh a temp file that already contains the template fails identically — the problem is the missing --body, not the file's contents.

What this does instead

Read the template here and fold it into the body we already send, with the issue URL appended:

## Summary

<!-- what changed -->

## Testing

https://linear.app/schpet/issue/CLI-113/...

The URL has to survive — it's what Linear matches on to attach the PR to its issue. Putting it last leaves the template's prose as the first thing a reviewer reads. No --template flag ever reaches gh, so every existing flag (--web, --draft, --base, --head, --title) keeps working unchanged.

Also fixed

• Config-relative paths broke outside the repo root. The loader finds <repo-root>/.linear.toml from any subdirectory, but its relative value was resolved against the cwd — so the documented pr_template = ".github/pull_request_template.md" worked at the root and failed everywhere below it. A config value now resolves against the config file that supplied it; a path typed on the command line stays cwd-relative, which is what a shell user expects.
• An invalid configured value was silently ignored. getOption() drops values that fail to parse, so pr_template = 123 would quietly produce a URL-only PR. Now a ValidationError, following the existing resolveIssueSort precedent — per CLAUDE.md, explicit input works or errors.
• Unusable templates fail loudly, naming the path: missing, directory, non-regular, unreadable. Silently falling back would ship a PR missing the content the user asked for.
• NUL bytes are rejected. Deno.readTextFile doesn't refuse binary input — it substitutes U+FFFD and keeps NULs, which Deno.Command then rejects with a bare TypeError that never names the file.

One deliberate surface change

#266's description suggests -T "" to override a configured default. That worked only because an empty string is falsy. It's now an explicit --no-template flag; -T "" gets cliffy's Missing value for option "--template", and --template x --no-template is rejected before anything runs.

Verification

573 tests pass; deno task check, deno lint, deno fmt --check clean. New test/commands/issue/issue-pull-request.test.ts covers body composition and every template-read rejection; test/config.test.ts covers strict validation and config-relative resolution.

QA'd against a gh shim capturing real argv: config default, --no-template, --web + template, missing/directory/binary/empty template, --template + --no-template, and a relative config path from a nested subdirectory — confirming the composed body and that --template never reaches gh.

Generated skill docs under skills/ are untouched; they're produced from an installed binary out of band and are already stale on trunk (they don't list --id from #268 either). CI only verifies they generate, not that they're current.

[linear]

CLI-113

[schpetbot]

Shipped in v2.6.0.

Follow-up to #266.

• The -T/--template flag and pr_template config option now actually reach GitHub; @maparent's surface was kept exactly as designed.
• The Linear issue URL is appended after the template, so the pull request stays linked to its issue.