feat(agent): close all post-fix remainders + golden eval harness (F.1-F.2, C.1-C.2, B.4-B.6, E.1-E.2)

Ships the 9 remaining post-fix items and a golden-conversation eval harness
that validates them against the live agent. All 4 evals pass.

SOUL.md (F.1, C.2, E.1):
- Consolidated three overlapping task-flow sections (MANDATORY TASK FLOW,
  'Every chat is a task', 'AFTER EVERY TASK: WRITE BACK') into one. ~50
  lines shorter. The operator's 'be more crisp' feedback.
- Added anti-patterns: don't re-execute on UI/sidebar complaints (C.2);
  don't re-run fleet-wide audits when same-day knowledge exists (E.1).
- Updated approval vocabulary in step 4 to match tasks.go (approved/yes/
  go/proceed/continue/ok/go ahead).

Tool-result strings (F.2):
- set_goal: tightened to 'Goal set. NEXT: pre-plan (read-only tools only).
  Then propose_plan. Do not call run.'
- update_plan_step: added '(Advance with update_plan_step + run; do not
  re-propose.)'

C.1 — completeTask rejects re-completion of a terminal session:
- Returns errTaskAlreadyComplete when status is already done/failed.
- The tool result directs: 'Task is already complete. Do not call
  complete_task again. If the operator pointed out a UI/sidebar
  inconsistency, fix it with update_plan_step...'

B.4 — Surface real model error text:
- chatWith's error event now includes finish_reason + refusal text:
  'Nomos returned an empty or unusable response (finish_reason=length).
  Retry or rephrase.' instead of generic 'empty response'.
- The resume-failed note already carried errText (B.3), which now has
  the real context.

B.5 — Back off between resume retries (4s, 8s):
- resumeSession now sleeps before attempts 1 and 2 (exponential backoff).
  A transient provider issue gets time to clear instead of 3 identical
  calls in 3 seconds.

B.6 — Don't persist the empty placeholder as a visible bubble:
- If a chat turn ends with no text and no tool calls (model empty-response'd
  and all retries failed), delete the placeholder row instead of persisting
  an empty bubble. The error was already streamed via done+error=true.

E.2 — list_lxcs last-audited hint:
- The list_lxcs result now includes last_audited_at — the most recent
  knowledge entry (tagged audit/update, or titled audit/update) linked
  via an 'about' edge. The agent can see 'nextcloud — last audited today'
  and skip re-running it.

Tool-call doubling bug fix (found by the eval harness):
- main.go + continue.go: the tool_use and tool_result events were both
  appending separate entries to the persisted tool_calls array, doubling
  every tool call in the transcript. Confirmed pre-existing (d9cdcee1,
  v0.3.x era). Fixed: tool_use creates the entry, tool_result merges the
  result into the same entry (matched by id). One entry per tool call.

Golden eval harness (cmd/nomos/eval/):
- A standalone Go program that loads YAML manifests of golden conversations
  + assertions, sends prompts to the chat endpoint, drains the SSE stream
  (keeping the agent's context alive), and scores structural assertions
  against the persisted transcript.
- 4 golden conversations covering: trivial read-only (degenerate case),
  plan + proceed (the original duplication bug), UI complaint (no re-exec),
  fleet audit (knowledge preferred over re-execution).
- Structural assertions only (tool-call sequences, plan steps, writeback,
  completion) — text quality is model-dependent and not scored.
- Run: go run ./cmd/nomos/eval -gateway http://localhost:8092 -manifest
  cmd/nomos/eval/evals/*.yaml  (~$0.10/run in OpenRouter credits).

Eval results (4/4 passed):
  trivial_readonly:              2 tool calls, no plan, no run
  plan_advances_on_proceed:     13 tool calls, propose_plan x1, writes back
  ui_complaint_no_rerun:        12 tool calls, propose_plan x1, writes back
  knowledge_preferred_over_rerun: 7 tool calls, search_knowledge x1, 0 run

Version 0.5.2 -> 0.5.3 (minor: eval harness + structural hardening).

cmd/nomos/tasks.go

@@ -181,7 +181,7 @@ func (a *agent) handleTaskTool(ctx context.Context, sessionID, name string, args
 		// without per-action approval. The operator approves the plan
 		// (propose_plan), not each individual run call.
 		a.store.openPlanWindow(ctx, sessionID)
-		return "Goal set: " + goal + ". Now do a PRE-PLAN: gather information with read-only tools (search_knowledge, get_entity, list_lxcs, get_relations), then call propose_plan with the full ordered steps. Do NOT call run yet.", true
+		return "Goal set: " + goal + ". NEXT: pre-plan (read-only tools only — search_knowledge, get_entity, list_lxcs, get_relations). Then propose_plan. Do not call run.", true
 
 	case "propose_plan":
 		raw, _ := args["steps"].([]any)
@@ -258,7 +258,7 @@ func (a *agent) handleTaskTool(ctx context.Context, sessionID, name string, args
 		if err := a.store.updatePlanStep(ctx, sessionID, seq, status, execID); err != nil {
 			return fmt.Sprintf("error updating step %d: %v", seq, err), true
 		}
-		return fmt.Sprintf("Step %d → %s", seq, status), true
+		return fmt.Sprintf("Step %d → %s. (Advance with update_plan_step + run; do not re-propose.)", seq, status), true
 
 	case "ask_operator":
 		prompt, _ := args["prompt"].(string)
@@ -314,6 +314,9 @@ func (a *agent) handleTaskTool(ctx context.Context, sessionID, name string, args
 			return "Refused: this session ran `run` against live targets (discovery) but did not call update_entity_attributes or create_relationship to persist what you learned. The knowledge graph will drift if you complete without writeback. Call update_entity_attributes for each entity you ran against (versions, states, counts, timestamps), and create_relationship for any edge you discovered, then call complete_task again. Outcome is held at 'executing' until you do.", true
 		}
 		if err := a.store.completeTask(ctx, sessionID, outcome, summary); err != nil {
+			if errors.Is(err, errTaskAlreadyComplete) {
+				return "Task is already complete. Do not call complete_task again. If the operator pointed out a UI/sidebar inconsistency, fix it with update_plan_step (reconcile step states) or summarize the panel in your reply — do not re-execute the work.", true
+			}
 			return fmt.Sprintf("error completing task: %v", err), true
 		}
 		result := fmt.Sprintf("Task marked %s: %s", outcome, summary)