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/store.go

@@ -158,6 +158,18 @@ func (s *store) updateMessage(ctx context.Context, id uuid.UUID, content json.Ra
 	return err
 }
 
+// deleteMessage removes a message row. Used by B.6: when a chat turn ends
+// with no text and no tool calls (the model empty-response'd and all
+// retries failed), the placeholder row is deleted instead of persisting an
+// empty assistant bubble — the error was already streamed to the frontend
+// via the 'done with error=true' event, so the operator sees it inline.
+func (s *store) deleteMessage(ctx context.Context, id uuid.UUID) {
+	if s == nil || id == uuid.Nil {
+		return
+	}
+	s.pool.Exec(ctx, `DELETE FROM agent_messages WHERE id = $1`, id)
+}
+
 // lastUserMessage returns the most recent user message text for a session,
 // or "" if none. Used to build a context-rich reconnect/resume note: instead
 // of a generic "report your state," the note can say "the operator's last
@@ -647,15 +659,38 @@ func (s *store) updatePlanStep(ctx context.Context, sessionID string, seq int, s
 	return nil
 }
 
+// errTaskAlreadyComplete is returned by completeTask when the session is
+// already in a terminal state (done/failed/partial). The agent sometimes
+// re-calls complete_task after a UI clarification (operator-reported
+// 2026-07-14) — without this guard, the re-completion produces duplicate
+// knowledge entries and erodes audit-log clarity. The caller translates this
+// into a directive tool result.
+var errTaskAlreadyComplete = errors.New("task already complete")
+
 // completeTask sets a task's terminal state, outcome, and one-line summary,
 // mirrors the outcome onto the task entity's attributes (so the board/graph
 // show it), and publishes task.status for the live context panel. outcome is
 // success|failure|partial; status is derived (failure → failed, else done).
+// Returns errTaskAlreadyComplete if the session is already terminal — the
+// agent must not re-complete a finished task.
 func (s *store) completeTask(ctx context.Context, sessionID, outcome, summary string) error {
 	if s == nil || sessionID == "" || sessionID == "ephemeral" {
 		return nil
 	}
 
+	// C.1: reject re-completion of an already-terminal session. The agent
+	// sometimes re-calls complete_task after a UI clarification ("the sidebar
+	// differs") — without this guard, the re-completion duplicates knowledge
+	// entries and produces a confusing audit trail.
+	var currentStatus string
+	if err := s.pool.QueryRow(ctx,
+		`SELECT status FROM agent_sessions WHERE id = $1`, sessionID).Scan(&currentStatus); err != nil {
+		// Session doesn't exist or query failed — let the rest of the
+		// function proceed; it'll fail safely on the UPDATE below.
+	} else if currentStatus == "done" || currentStatus == "failed" {
+		return errTaskAlreadyComplete
+	}
+
 	// Auto-cancel any executions still in pending_approval/approved/queued
 	// state for this session — preventing orphaned approvals (observed in
 	// production: 4 approvals left open after session completed).