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).

internal/mcp/server.go

@@ -485,7 +485,7 @@ func newServer(pool *db.Pool, agentID uuid.UUID) *mcp.Server {
 
 	// ─── Phase 5: operational MCP tools ──────────────────────────────
 
-	register(&mcp.Tool{Name: "list_lxcs", Description: "List all LXC containers with ID, host, IP, and state. Pass state=\"active\" to exclude destroyed/deprecated containers.",
+	register(&mcp.Tool{Name: "list_lxcs", Description: "List all LXC containers with ID, host, IP, state, and last-audited hint. Pass state=\"active\" to exclude destroyed/deprecated containers. The last_audited_at column shows the most recent knowledge entry (investigation or document tagged audit/update) linked via an 'about' edge — use it to skip re-running `run` against LXCs that were already audited recently.",
 		InputSchema: objSchema(
 			prop{"state", "string", "Optional: filter by entity state (active, destroyed, …)"},
 		),
@@ -499,7 +499,18 @@ func newServer(pool *db.Pool, agentID uuid.UUID) *mcp.Server {
 			SELECT e.slug, e.name, e.attributes->>'pve_id' AS pve_id,
 			       e.attributes->>'lan_ip' AS lan_ip,
 			       e.state,
-			       st.health, st.last_check_at
+			       st.health, st.last_check_at,
+			       (SELECT MAX(k.created_at)
+			        FROM relationships r
+			        JOIN knowledge_entities k ON k.entity_id = r.source_id
+			        WHERE r.target_id = e.id
+			          AND r.type = 'about'
+			          AND r.valid_to IS NULL
+			          AND (k.tags @> ARRAY['audit']::text[]
+			            OR k.tags @> ARRAY['update']::text[]
+			            OR k.title ILIKE '%audit%'
+			            OR k.title ILIKE '%update%')
+			       ) AS last_audited_at
 			FROM entities e
 			LEFT JOIN entity_status st ON st.entity_id = e.id
 			WHERE e.type = 'lxc'