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  to the CURRENT plan after a re-plan, instead of resurrecting a superseded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  returns errPlanStepNotFound (never touches a superseded generation).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

web/src/lib/stores/workspace.ts

@@ -93,12 +93,30 @@ function scheduleSessionsRefresh() {
   refreshTimer = setTimeout(() => loadSessions(), 300)
 }
 
+// A dropped plan-step event is one that matched no step on screen —
+// historically a silent return, which made a disagreeing backend look like a
+// dead UI (the panel froze showing pending steps while work happened
+// elsewhere). Surface it so the next divergence is visible. Exported so a
+// debug surface (or a test) can read the count.
+export let droppedPlanStepEvents = 0
+export function resetDroppedPlanStepEvents(): void {
+  droppedPlanStepEvents = 0
+}
+
 function applyPlanStepEventTo(ws: WorkspaceState, data: PlanStepEventData) {
   const stepID = data?.step_id
   const seq = data?.seq
   ws.planSteps.update((steps) => {
     const i = steps.findIndex((s) => (stepID && s.id === stepID) || (seq != null && s.seq === seq))
-    if (i === -1) return steps
+    if (i === -1) {
+      droppedPlanStepEvents++
+      console.warn('plan step event matched no step on screen', {
+        stepID,
+        seq,
+        status: data?.status
+      })
+      return steps
+    }
     const next = [...steps]
     next[i] = {
       ...next[i],