git repo discovery: root walk, worktree dedup

[Saber5656]

Title

git repo discovery: root walk, worktree dedup

Summary

Implement repository discovery: depth-limited walk of configured roots,
.git dir/file detection, exclusion rules, symlink safety, and worktree
deduplication via git rev-parse --git-common-dir (DESIGN.md §7.2
"Discovery").

Context

The user's environment keeps repos under ~/dev and uses git worktrees
heavily (1 task = 1 worktree policy). Naive discovery would list a main repo
and each of its worktrees as separate repos and duplicate every commit in
reports. The common-dir dedup rule here is what prevents that.

Scope

• internal/source/gitsrc/discover.go + tests (fixture repos built in
  t.TempDir() by test helpers).

Detailed Requirements

1. type Repo struct { WorkDir string; CommonDir string; Name string } —
   Name = base name of the canonical work dir.
2. Discover(ctx context.Context, cfg config.SourcesGit, runner Runner) ([]Repo, []model.Warning):
   • Walk each root from cfg.GitRoots() (tilde-expanded by config
     accessors): breadth-first, max depth cfg.MaxDepth (root itself =
     depth 0).
   • Skip (do not descend into): directories whose base name is in
     exclude_dirs; all dot-directories (.foo) except that a .git
     entry marks its parent as a repo; symlinked directories (Lstat
     check — never follow, cycle prevention).
   • Repo detection: directory containing .git (dir or file). On
     detection, record candidate and do NOT descend further into it.
   • Missing/non-directory root → warning git_root_missing (already part
     of the canonical DESIGN §13 taxonomy; the constant exists in
     internal/model from issue 03).
   • Explicit cfg.Repos paths join the candidate list (missing path →
     git_root_missing warning naming it).
3. Dedup: for each candidate run
   git --no-optional-locks -C <dir> rev-parse --path-format=absolute --git-common-dir via the injected Runner (interface
   Run(ctx, dir string, args ...string) (stdout string, err error) — real
   impl in issue 12, test impl fake). Canonical key = cleaned common-dir
   path. First candidate wins per key; candidates whose rev-parse fails →
   warning git_repo_failed (skip). WorkDir of the kept entry: if the
   common dir is <X>/.git, use X; else (detached/odd layouts) keep the
   candidate dir.
4. Deterministic output: sort Repos by Name then WorkDir.
5. git binary absence is issue 12's Runner concern: Discover surfaces
   it as a single git_binary_missing warning and returns zero repos
   (detected on first Run error of type ErrGitNotFound).
6. Walk must be allocation-sane on big trees: use os.ReadDir, check
   ctx.Err() per directory.

Acceptance Criteria

• Fixture tree: root with 2 normal repos at depth 1–2, one repo below
  max_depth (excluded), one under node_modules (excluded), one
  symlinked dir (not followed), one plain dir (not a repo) → exactly the
  2 expected repos.
• Worktree fixture: main repo + git worktree add sibling → ONE Repo,
  Name = main repo dir name (helper builds real worktrees with the
  system git).
• Explicit repos entry pointing at a worktree of an already-discovered
  main repo → still one Repo (dedup across discovery+explicit).
• Missing root and missing explicit repo → git_root_missing warnings,
  others unaffected.
• Fake runner returning error for one candidate → that candidate
  skipped with git_repo_failed, others kept.
• Determinism: shuffled directory creation order still yields identical
  sorted output.

Validation

go test -race -cover ./internal/source/gitsrc/ in PR (helper-built
fixtures; requires system git on the test machine — CI has it).

Dependencies

01, 03.

Non-goals

Commit extraction (12). Submodules: because the walk never descends past a
detected repo boundary, submodules inside a discovered repo are NOT
enumerated (their commits belong to their own repos; users who want them add
the submodule path to repos explicitly). A submodule checkout sitting
directly under a root (unusual) is treated as an ordinary repo candidate.
Bare repos: not supported in v1 (no worktree = no daily-work relevance);
rev-parse-based dedup keeps them from duplicating anything.

Design References

• docs/DESIGN.md §7.2 (Discovery), §13
• docs/decisions/ADR-003-read-only-sources-and-stateless-runs.md

---

Source of truth: docs/issues/11-git-repo-discovery.md (PR #1, branch docs/v1-design). If this issue and the repo docs disagree, the docs win. Execution order and dependencies: docs/ISSUE_PLAN.md (this is issue 11 of 33).