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  to the CURRENT plan after a re-plan, instead of resurrecting a superseded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  returns errPlanStepNotFound (never touches a superseded generation).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/main.go

@@ -483,7 +483,8 @@ func handleSessionDetail(w http.ResponseWriter, r *http.Request, st *store, a *a
 	if len(parts) == 2 && r.Method == http.MethodGet {
 		switch parts[1] {
 		case "plan":
-			steps, err := st.getPlanSteps(r.Context(), id)
+			all := r.URL.Query().Has("all") && r.URL.Query().Get("all") != "0" && r.URL.Query().Get("all") != "false"
+			steps, err := st.getPlanSteps(r.Context(), id, all)
 			if err != nil {
 				http.Error(w, err.Error(), 500)
 				return