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/eval/sse.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+	"bufio"
+	"encoding/json"
+	"io"
+	"strings"
+)
+
+// sseReader parses a text/event-stream into a sequence of JSON events.
+// Each event is one or more "data: " lines; the lines are concatenated
+// and parsed as a single JSON object. Blank lines separate events.
+type sseReader struct {
+	r *bufio.Reader
+}
+
+func newSSEReader(r io.Reader) *sseReader {
+	return &sseReader{r: bufio.NewReader(r)}
+}
+
+func (s *sseReader) next() (map[string]any, error) {
+	var data strings.Builder
+	for {
+		line, err := s.r.ReadString('\n')
+		if err != nil {
+			if err == io.EOF && data.Len() > 0 {
+				return parseEvent(data.String())
+			}
+			return nil, err
+		}
+		line = strings.TrimRight(line, "\r\n")
+		if line == "" {
+			if data.Len() > 0 {
+				return parseEvent(data.String())
+			}
+			continue // blank line, no event buffered yet
+		}
+		if strings.HasPrefix(line, "data: ") {
+			data.WriteString(strings.TrimPrefix(line, "data: "))
+		} else if strings.HasPrefix(line, "data:") {
+			data.WriteString(strings.TrimPrefix(line, "data:"))
+		}
+	}
+}
+
+func parseEvent(s string) (map[string]any, error) {
+	var ev map[string]any
+	if err := json.Unmarshal([]byte(s), &ev); err != nil {
+		return nil, err
+	}
+	return ev, nil
+}