End the implementer's trajectory when Orchestra reopens the phase

Run 20 showed the leak precisely: the reopen rotated the session, and 31
seconds later the planning session that replaced it recorded phase-2 of the
plan being replaced. The implementer's .orchestra/plan-progress.json had
outlived it in the worktree, and the successor executed it.

Two changes. Phase verification now belongs to the implement phase and is
refused anywhere else, which covers every worker and every path rather than
the one that produced it. The rotation also drops the ended session's
request files, so a successor never inherits work from a trajectory
Orchestra has already invalidated.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CVbaKucEYBjMqVeUgJUsc1

internal/operations/planprogress.go

@@ -43,6 +43,13 @@ func PlanPhaseCommands(s *store.Store, project registry.Project, taskID, phaseID
 	if t.PlanRef == "" {
 		return workphase.PlanPhase{}, fmt.Errorf("%w: this task has no accepted plan", ErrPlanPhase)
 	}
+	// Verification is implementation work. A request that arrives in another
+	// phase belongs to a trajectory Orchestra has already ended: run 20's
+	// reopened planning session executed the implementer's leftover request
+	// and recorded a phase of a plan that was being replaced.
+	if current(t) != domain.WorkPhaseImplement {
+		return workphase.PlanPhase{}, fmt.Errorf("%w: phase verification belongs to the implement phase, and this task is in %s", ErrPlanPhase, current(t))
+	}
 	raw, err := s.Artifact(t.PlanRef)
 	if err != nil {
 		return workphase.PlanPhase{}, fmt.Errorf("read accepted plan: %w", err)