fix(agent): task completion safety net — stop tasks sticking at Running

Implements fixes 1-3 of plans/2026-07-11-task-completion-safety-net.md.
Confirmed live that 50/50 production sessions never reached a terminal
status because the model almost never calls complete_task, even for
trivial single-tool Q&A turns SOUL.md explicitly calls out as needing it.

- Inline safety net (agent.go): a session that never called set_goal never
  framed itself as a structured task, so its first plain-text turn-end IS
  the task ending — auto-complete it there instead of leaving status stuck
  at its creation default forever.
- Idle sweep (continue.go, new completion_nudges column): goal-bearing
  sessions that stall get one nudge, then auto-close with outcome=partial
  if the nudge goes unanswered, mirroring the pattern resumeSession already
  uses for a different stuck-session failure mode.

Fix 4 (backfill of the 50 already-stuck live sessions) is deliberately
separate — deferred until this is deployed and verified live, per the
plan's implementation order.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

cmd/nomos/main.go

@@ -80,6 +80,11 @@ func main() {
 		// from failures) without the operator ticking it forward each step.
 		safego.Go("nomos:continuation-worker", func() { nAgent.runContinuationWorker(ctx) })
 
+		// Idle sweep for stalled goal-bearing tasks (fix 2+3 of
+		// plans/2026-07-11-task-completion-safety-net.md) — a coarser,
+		// slower-ticking counterpart to the continuation worker above.
+		safego.Go("nomos:idle-sweep-worker", func() { nAgent.runIdleSweepWorker(ctx) })
+
 		safego.Go("nomos:mcp-pool-sweeper", func() {
 			ticker := time.NewTicker(5 * time.Minute)
 			defer ticker.Stop()