feat(lib): add gh-stack as a stack provider

[lettertwo]

This PR adds gh-stack as a provider that list, find, routing, and changeset assembly can read.
Read-only: nothing here writes a gh-stack file or links a worktree.

git-workon-lib/src/stack/gh_stack.rs parses gh-stack's JSON into a StackMetadata.
StackModel::detect recognizes a gh-stack repo, and workon.stackModel accepts gh-stack.

Detection checks Graphite first. .graphite_repo_config comes from a deliberate repo-wide
gt init, while a gh-stack file can appear as a side effect of one gh stack add in a single
worktree.

Two read rules differ from Graphite's. A truncated read retries 3 times and then skips the file,
since upstream's os.WriteFile truncates in place and a partial file is the expected steady state
during a concurrent gh stack run. schemaVersion > 1 is a hard error and is never retried. Both
in docs/adr/028-provider-neutral-stack-metadata.md.

Reads never invoke gh. Changeset.title is None under gh-stack as a result.

git-workon-fixture/src/fixture_builder.rs gains gh_stack, gh_stack_at, raw_gh_stack, and
four predicates. Worth reading first, since the gh-stack tests are written against them.

Testing: 607 tests, clippy clean.

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