[prop4n]

Increment 1 of the observability work: show which Git commit the cluster is
reconciled against.

What it does

• The Git source records the resolved commit hash on each sync and exposes it via
  Commit(). A local (non-Git) source has none.
• The engine reads it through an optional interface{ Commit() string } and puts
  it on the status snapshot (Snapshot.Commit), so nothing else changes for the
  local source.
• The overview status toolbar shows the short commit (full hash on hover).

Tests

• Engine records the commit from a committing source; empty for a plain source.
• Full Go + vitest suites, golangci-lint (v2) and gofmt clean.

Next (same design doc)

• Increment 2: per-resource history persisted in SQLite (events on change +
  applied action, with the commit).
• Increment 3: daemon logs in the UI via an in-memory ring buffer over SSE.

Independent of PR #7 (templates); both touch engine.go but in different spots.