fix(generate): chmod on ActionUnchanged + close Plan 12 test gaps (Plan 13)

[LastStep]

Summary

• writeFileChmod now re-applies its declared perm when the inner writeFile returns ActionUnchanged. Previously, a sensor script whose exec bit had been stripped externally would stay non-executable across bonsai update runs once Plan 12's content-equality short-circuit took effect.
• TestWriteResultSummary now asserts the unchanged count (previously discarded via _).
• New TestWorkspaceClaudeMDUnchangedShortCircuit covers the short-circuit at internal/generate/generate.go:829-833 — second call asserts ActionUnchanged and verifies mtime is unchanged (file was back-dated first to make rewrites detectable on coarse-mtime filesystems).
• New TestWriteFileChmodRestoresPermOnUnchanged regression test: create at 0755 → strip to 0644 externally → re-run with identical content → assert ActionUnchanged + mode restored to 0755.

Plan: station/Playbook/Plans/Active/13-actionunchanged-followups.md. Follow-up to Plan 12 (PR #20).

Test plan

• `go build ./...`
• `go vet ./...`
• `go test ./...` — all pass
• `gofmt -s -l cmd/ internal/ main.go` — empty
• Manual: in a dogfooded workspace, `chmod -x station/agent/Sensors/status-bar.sh && bonsai update` — expect "Up to date" panel AND exec bit restored
• Manual: two consecutive `bonsai update` runs on a clean workspace — second shows "Up to date" with no mtime churn

🤖 Generated with Claude Code