fix(nomos): generation-relative plan seq + real activity timestamps

The plan recorded false history after a re-plan and the activity panel
showed fabricated, churning timestamps. Two bugs compounding on one event
stream.

Plan drift (P0.1):
- proposePlan seq is now 1..N per generation; (session,generation,seq) is
  the addressing key. The model's 1-based update_plan_step calls always map
  to the CURRENT plan after a re-plan, instead of resurrecting a superseded
  `replaced` row as done while the live work went unrecorded.
- updatePlanStep resolves against MAX(generation); a stale/out-of-range seq
  returns errPlanStepNotFound (never touches a superseded generation).
- getPlanSteps returns only the current generation by default; ?all=true
  keeps the audit/eval view (plan_generations assertion).
- completeTask auto-close scopes to the current gen, stamps started_at, and
  emits one plan.step.finished per closed step so the panel converges
  instead of freezing on "running" after completion (P1.1).
- propose_plan result enumerates step seqs; writeback detector matches
  "write back"/"writeback"/"upsert_knowledge" so a natural-language final
  step isn't doubled (P1.2).
- migration 029 renumbers existing seq per generation + unique index.

Activity panel (P0.2 / P1.1, web):
- computeActivityLog uses the real message created_at for tool calls; live
  entries fall back to wall-clock frozen on first sight, killing the 3s
  poll churn. Steps use real started_at.
- dropped plan-step events warn + count instead of a silent no-op.

Tests: TestProposePlan updated; + generation-relative-seq and auto-close
event-emission regression tests; + web activity purity/timestamp tests.

VERSION: 0.14.0 -> 0.14.1

cmd/nomos/eval/main.go

@@ -319,8 +319,10 @@ func fetchTranscript(ctx context.Context, gateway, sid string) (transcript, sess
 	}
 	// Fetch the plan (steps with generation numbers) for the
 	// plan_generations assertion. A 404 or empty response is fine — a
-	// pure-DB Q&A with no propose_plan has no plan.
-	if planResp, perr := http.Get(gateway + "/sessions/" + sid + "/plan"); perr == nil {
+	// pure-DB Q&A with no propose_plan has no plan. ?all=true returns every
+	// generation so the assertion can count them (the default view returns
+	// only the current generation).
+	if planResp, perr := http.Get(gateway + "/sessions/" + sid + "/plan?all=true"); perr == nil {
 		if planResp.StatusCode == 200 {
 			pb, _ := io.ReadAll(planResp.Body)
 			_ = json.Unmarshal(pb, &t) // fills t.PlanSteps via "steps" field