fix(agent): reopenSession replaces plan steps for executing sessions too

A follow-up on an executing session (first turn didn't complete_task) is
still a new direction — the old plan's steps must not block the new one.
Previously reopenSession was a no-op for executing sessions, leaving done
steps that caused errPlanInFlight on the next propose_plan call.

cmd/nomos/store.go

@@ -478,11 +478,16 @@ func (s *store) setGoal(ctx context.Context, sessionID, goal string) error {
 //     audit trail survives. proposePlan's anyStarted check excludes
 //     `replaced` (see proposePlan), so the next propose_plan takes the
 //     fresh-generation path rather than being refused with errPlanInFlight.
-//  2. Clears outcome/summary so the panel doesn't show the old result.
+//     This runs for BOTH terminal and executing sessions — a follow-up on
+//     an executing session (first turn didn't complete_task) is still a
+//     new direction, and the old plan's steps must not block the new one.
+//  2. If the session was terminal, clears outcome/summary and flips status
+//     to `executing` so the panel doesn't show the old result.
 //  3. Stamps last_active_at.
 //
 // Returns true if the session was actually reopened (was terminal), false if
-// it was already active (no-op — the follow-up is just a continuation).
+// it was already active (the plan steps are still replaced — the new
+// direction needs a fresh plan — but the status doesn't change).
 func (s *store) reopenSession(ctx context.Context, sessionID string) bool {
 	if s == nil || sessionID == "" || sessionID == "ephemeral" {
 		return false
@@ -492,12 +497,14 @@ func (s *store) reopenSession(ctx context.Context, sessionID string) bool {
 		`SELECT status FROM agent_sessions WHERE id = $1`, sessionID).Scan(&currentStatus); err != nil {
 		return false
 	}
-	if currentStatus != "done" && currentStatus != "failed" {
-		return false
-	}
+	// Always replace prior plan steps — the follow-up is a new direction
+	// regardless of whether the prior turn completed. Without this, an
+	// executing session with done steps would block proposePlan with
+	// errPlanInFlight.
 	s.pool.Exec(ctx,
 		`UPDATE session_plan_steps SET status = 'replaced', finished_at = COALESCE(finished_at, now()) WHERE session_id = $1 AND status <> 'replaced'`,
 		sessionID)
+	if currentStatus == "done" || currentStatus == "failed" {
 		s.pool.Exec(ctx,
 			`UPDATE agent_sessions SET status = 'executing', outcome = NULL, summary = NULL, last_active_at = now() WHERE id = $1`,
 			sessionID)
@@ -505,6 +512,9 @@ func (s *store) reopenSession(ctx context.Context, sessionID string) bool {
 			"info", "nomos", sessionID, map[string]any{"prior_status": currentStatus})
 		return true
 	}
+	s.pool.Exec(ctx, `UPDATE agent_sessions SET last_active_at = now() WHERE id = $1`, sessionID)
+	return false
+}
 
 // planStepInput is one step as the agent proposes it.
 type planStepInput struct {