feat: completion-enforcement loop (fallback, proof, emit-model, auditor)

Close the loop so dispatched work actually completes on bl:

- on_no_result fallback in followup-sweeper (op + job forms via
  exec-constrained registry / job-dispatch), seeded on the three
  autonomy-pulse jobs; fallback_due() dedupes the gravity path
- gravity.py: add __main__ entry (loop-remediator.timer was a no-op),
  300s re-arm budget, fallback firing + stamp/skip logic
- harvester: proof-of-result followups (result_has_evidence),
  acted-variant NACK, emit-model tool-hint wording
- envelope: RESPONSE RULE states the emit model (agents EMIT
  directives verbatim; runtime executes; works from bare containers)
- completion-audit.py + systemd 15-min timer: per-family funnel,
  swarm drain, followup backlog; digest DM when degraded, 6h heartbeat
- tests/test_completion.py (29 tests), JOB-SPEC.md docs

Tests: 67/67 focused green (completion + tool_calls).

systemd/completion-audit.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Run completion auditor every 15 minutes
+
+[Timer]
+OnCalendar=*:4/15
+Persistent=true
+
+[Install]
+WantedBy=timers.target