Spec Kitty

Mission Run: Upgrade idempotency — no divergent per-worktree metadata stamps

A repeat 'spec-kitty upgrade' on an already-current project must not wedge in-flight coord missions.

When an operator (or a teammate who pulled an already-upgraded main) re-runs 'spec-kitty upgrade', the CLI stamps each live worktree with its own last_upgraded_at and auto-commits it per branch, so main, coord and lane branches diverge on one bookkeeping line. Every in-flight coord mission then becomes undrivable: 'implement' fails the dependency-lane auto-merge and 'merge' refuses the lane as stale — while both upgrades exit 0 'already up to date'. This mission makes upgrade idempotent for live worktrees and prevents spec-kitty's own bookkeeping from wedging the mission.