fix(agent): refuse plan re-proposal + emit done on error (close divergence chain)

Operator-reported bug: on 'proceed with the rest' the agent re-proposed the
plan, duplicating it in the sidebar. Root cause was a three-bug chain, not
one bug:

1. Trigger — model empty-response on 'proceed' (approval vocabulary didn't
   list 'proceed', so the agent wasn't sure it was approved and no-op'd).
2. Amplifier — chatWith emitted 'error' without 'done' on empty response
   (agent.go:370). The frontend's onComplete saw !receivedDone and
   misclassified the model failure as a network disconnect, calling
   handleDisconnect -> resumeSession.
3. Divergence — the reconnect note was generic ('report your state'), so
   the agent re-proposed + re-executed instead of advancing the plan.

Fixes (shipped, e2e-validated against the live agent on oikos-nomos-1):

- A.2: proposePlan refuses re-proposal once a step has started (returns
  errPlanInFlight). Drops the append-mode safety net (commit 5384499) that
  was the direct source of the sidebar duplication. The agent must advance
  with update_plan_step + run; the tool result directs it.
- A.1: proposePlan sets the 'generation' column on INSERT (migration 020
  added the column + frontend grouping, but the INSERT never wired it).
- A.3: propose_plan tool description restated as a crisp contract (ONCE,
  STOP and wait, REFUSES once a step started, advance with update_plan_step).
- F.3: approval vocabulary expanded to approved/yes/go/proceed/continue/ok/
  go ahead; propose_plan result string tightened to an imperative.
- B.1: chatWith emits 'done' after 'error' on every terminal path via a new
  emitError helper. The frontend now treats model errors as ended (not
  disconnected), so no auto-reconnect -> resumeSession fires.
- B.2: reconnect/resume note carries the operator's last message + an
  explicit 'advance the plan, do NOT call propose_plan again' directive when
  a plan is in flight. Wired into all 4 resume entry points (reconnect,
  /resume, idle-sweep, question-answer) via enrichResumeNote.
- B.3: resumeSession escalates the recovery note across its 3 attempts (final
  retry: 'pick the lowest-pending step, mark it running, call run — do that
  now') instead of 3 identical notes -> 3 identical empties.

Verification: TestProposePlan_RefuseInFlight replaces TestProposePlan_
AppendVsReplace. e2e conversations against the rebuilt container:
  conv2 ('proceed with the rest') -> 0 propose_plan calls, plan stayed at
    3 steps (was 6+ before), update_plan_step x5 + run x2 + complete_task.
  conv3 (full plan, 'go ahead') -> apt-get update on lxc:dns auto-ran under
    the plan window, update_entity_attributes writeback, clean complete_task.
  nomos logs show zero reconnect/resume entries for the plan-proposing
    sessions (the three-bug chain is closed).

Remaining (not in this commit): D.1 refuse complete_task without writeback
(next blocker), C.1/C.2, F.1/F.2 SOUL.md consolidation, B.4-B.6, E.1/E.2.
See plans/2026-07-14-post-fix-session-remainders.md.

Also: re-audit 2026-07-10-general-gated-execution.md — request_execution enum
retirement (60effcb) closes item 9; only auto-act revival (item 10) remains.

Version 0.4.1 -> 0.5.0 (minor: new structural behavior, not a bugfix).

cmd/nomos/agent.go

@@ -187,9 +187,30 @@ func (a *agent) chat(ctx context.Context, sessionID, message string, emit func(a
 func (a *agent) chatWith(ctx context.Context, sessionID, message, systemInject string, emit func(agentEvent)) {
 	correlationID := uuid.New().String()
 
+	// emitError emits an error event followed by a done event. The done
+	// event is CRITICAL on every terminal path: the frontend's
+	// onComplete handler (chat.ts) treats a missing `done` as a severed
+	// network connection and triggers an auto-reconnect → resumeSession.
+	// Before this fix, a model empty-response (the most common case here)
+	// returned without `done`, was misclassified as a network drop, and
+	// the reconnect logic re-invoked the agent with a generic "report
+	// your state" note — which caused the agent to re-propose the plan
+	// and duplicate it in the sidebar (operator-reported 2026-07-14).
+	// Every error return below must go through emitError so the frontend
+	// shows the error inline instead of silently reconnecting.
+	emitError := func(data string) {
+		emit(agentEvent{Type: "error", Data: data, SessionID: sessionID})
+		emit(agentEvent{Type: "done", Data: map[string]any{
+			"session_id":     sessionID,
+			"correlation_id": correlationID,
+			"iterations":     0,
+			"error":          true,
+		}, SessionID: sessionID})
+	}
+
 	tools, err := a.buildTools(sessionID)
 	if err != nil {
-		emit(agentEvent{Type: "error", Data: fmt.Sprintf("build tools: %v", err), SessionID: sessionID})
+		emitError(fmt.Sprintf("build tools: %v", err))
 		return
 	}
 
@@ -345,7 +366,7 @@ func (a *agent) chatWith(ctx context.Context, sessionID, message, systemInject s
 					slog.Warn("nomos: llm stream error, retrying", "error", err, "attempt", attempt+1, "session", sessionID)
 					continue
 				}
-				emit(agentEvent{Type: "error", Data: fmt.Sprintf("llm: %v", err), SessionID: sessionID})
+				emitError(fmt.Sprintf("llm: %v", err))
 				return
 			}
 			if len(acc.Choices) == 0 {
@@ -353,7 +374,7 @@ func (a *agent) chatWith(ctx context.Context, sessionID, message, systemInject s
 					slog.Warn("nomos: no choices in response, retrying", "attempt", attempt+1, "session", sessionID)
 					continue
 				}
-				emit(agentEvent{Type: "error", Data: "no choices in response", SessionID: sessionID})
+				emitError("no choices in response")
 				return
 			}
 
@@ -367,7 +388,7 @@ func (a *agent) chatWith(ctx context.Context, sessionID, message, systemInject s
 							"content_len", len(msg.Content))
 						continue
 					}
-					emit(agentEvent{Type: "error", Data: "Nomos returned an empty or unusable response — please retry.", SessionID: sessionID})
+					emitError("Nomos returned an empty or unusable response — please retry.")
 					return
 				}
 			}