audit follow-up PR-B: cousin-shape symmetry (F-5, F-14)

[oubiwann]

Summary

Closes the two CONSIDER findings the 0015 plan grouped as PR-B — both about the diagnostic chain matching the actual git diff exec invocation. Single commit per the plan's PR-B scope.

Findings closed

F-5 — *GitDiffError cousin-shape parity

*GitDiffError (internal/cli/errors.go) now carries a Dir string field next to Cmd []string, mirroring *golist.ExitError's shape exactly. classifyGitDiffError's third parameter is dir string (was _); the caller at internal/cli/cli.go passes cfg.root which is now stored on *GitDiffError.Dir instead of being discarded.

Test addition: TestClassifyGitDiffError/exec_exit_error in internal/cli/seam_test.go now asserts ge.Dir == "/m".

F-14 — return argv from the git-diff seam

gitDiffResult (internal/cli/seam.go) grew an argv []string field. defaultRunGitDiff constructs argv once and uses it both for the actual exec.CommandContext invocation and as the returned diagnostic argv — the two paths can no longer drift. The synthetic argv reconstruction at internal/cli/cli.go's loadChangeSet is removed; the call site now reads classifyGitDiffError(res.err, res.argv, cfg.root, res.stderr, ctx).

Tests that synthesize gitDiffResult without setting argv produce nil Cmd in the diagnostic — acceptable for the existing tests (none assert on Cmd); a future test that needs argv just sets it on the synthetic result.

Diff scope

5 files modified, 47 insertions / 9 deletions. Plan estimated ~30 lines.

File	Change
internal/cli/errors.go	+ Dir string field on *GitDiffError + doc
internal/cli/seam.go	+ argv []string on gitDiffResult; defaultRunGitDiff returns argv; classifyGitDiffError takes dir string and stores it
internal/cli/cli.go	drop synthetic argv reconstruction; pass res.argv
internal/cli/seam_test.go	new ge.Dir == "/m" assertion
docs/dev/0014-go-quality-audit.md	PR-B closure walk appended at the bottom

Test plan

• make check (build + lint via golangci-lint + go test -race -count=1 ./...)
• Confirm *GitDiffError shape matches *golist.ExitError (grep -n 'Dir ' internal/cli/errors.go pkg/golist/errors.go)
• Confirm the synthetic argv reconstruction at the old cli.go:269 is gone (grep -n 'git diff --name-only' internal/cli/cli.go — should only show the doc / help text, not a constructed argv)
• Read the F-14 closure paragraph in docs/dev/0014-go-quality-audit.md

Stacking

Base: audit/0014-go-quality-audit (PR #15). After PR #15 merges, this PR's base can be retargeted to main.

Parallel to PR-A (PR #16, doc pass closing 14 findings + PROMPT-1). PR-A and PR-B do not conflict in code (different files / different lines); each updates docs/dev/0014-go-quality-audit.md at a different anchor (PR-A above the existing ## Closure heading; PR-B at the file end) so the doc edits don't conflict either.

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