fix(agent): move plan step replacement from reopenSession to setGoal

reopenSession was replacing plan steps on every follow-up message —
including approvals ('go ahead') — which destroyed the plan the operator
just approved, leaving the agent unable to track step progress and looping
run calls until maxIterations.

Fix: setGoal is the explicit signal for 'new sub-task' (the agent calls
it at the start of each follow-up direction). Step replacement now happens
there, not in reopenSession. An approval ('go ahead') does NOT call
set_goal, so the plan stays intact and the agent can execute + complete
it.

cmd/nomos/store.go

@@ -452,10 +452,24 @@ func (s *store) taskEntityPtr(ctx context.Context, sessionID string) *uuid.UUID
 // intermediate state was removed (2026-07-14) — it was indistinguishable from
 // `active` to the operator and caused sessions to appear stuck when the agent
 // called set_goal but never propose_plan (observed in production).
+//
+// P2 (2026-07-15): setGoal also replaces any prior plan steps (from a
+// previous sub-task or an incomplete first turn) as `replaced`, clearing the
+// way for a fresh propose_plan. This is the ONLY place step replacement
+// happens — not in reopenSession — because set_goal is the explicit signal
+// for "new sub-task." An approval ("go ahead") does NOT call set_goal, so it
+// won't destroy the plan the operator just approved.
 func (s *store) setGoal(ctx context.Context, sessionID, goal string) error {
 	if s == nil || sessionID == "" || sessionID == "ephemeral" {
 		return nil
 	}
+	// Replace any prior plan steps (done/running/pending/...) as `replaced`.
+	// The rows are kept for the generation counter + audit trail; proposePlan
+	// excludes `replaced` from its in-flight check, so the next propose_plan
+	// takes the fresh-generation path.
+	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 _, err := s.pool.Exec(ctx,
 		`UPDATE agent_sessions SET goal = $2, status = 'executing', last_active_at = now() WHERE id = $1`,
 		sessionID, goal); err != nil {
@@ -469,25 +483,17 @@ func (s *store) setGoal(ctx context.Context, sessionID, goal string) error {
 // reopenSession flips a terminal (done/failed) session back to `executing`
 // so a follow-up message can start a new sub-task — the iteration path
 // (P2, 2026-07-15). Without this, a completed session stays `done` forever
-// and propose_plan refuses the new sub-task with errPlanInFlight because the
-// prior plan's steps are all `done` (status <> 'pending'). reopenSession:
+// and the panel shows a stale result.
 //
-//  1. Marks all existing session_plan_steps as `replaced` (a status already
-//     recognized by updatePlanStep's stamp switch). The rows are KEPT — the
-//     generation column preserves which plan they belonged to, and the
-//     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.
-//     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.
+// reopenSession ONLY flips the status + clears outcome/summary. It does NOT
+// touch plan steps — that's `setGoal`'s job (see below). The reason: not
+// every follow-up is a new sub-task. An approval ("go ahead") is a
+// continuation of the current plan, and replacing its steps would destroy
+// the plan the operator just approved. `set_goal` is the explicit signal for
+// "new sub-task," so step replacement happens there, not here.
 //
 // Returns true if the session was actually reopened (was terminal), false if
-// it was already active (the plan steps are still replaced — the new
-// direction needs a fresh plan — but the status doesn't change).
+// it was already active (no-op).
 func (s *store) reopenSession(ctx context.Context, sessionID string) bool {
 	if s == nil || sessionID == "" || sessionID == "ephemeral" {
 		return false
@@ -497,14 +503,9 @@ 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
 	}
-	// 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" {
+	if currentStatus != "done" && currentStatus != "failed" {
+		return false
+	}
 	s.pool.Exec(ctx,
 		`UPDATE agent_sessions SET status = 'executing', outcome = NULL, summary = NULL, last_active_at = now() WHERE id = $1`,
 		sessionID)
@@ -512,9 +513,6 @@ 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 {