feat(cron): wire on_jobs_changed, cron.chronos config, docs + agent↔NAS contract

Phase 4F (F.1 + F.2 + F.3, agent side). F.4 is the operator-run live smoke
(needs a NAS deployment); recorded in the PR, not code.

F.1 — on_jobs_changed wiring:
- cron/scheduler.py: _notify_provider_jobs_changed() — resolve the active
  provider, call on_jobs_changed(), swallow errors. Lives in scheduler.py (not
  jobs.py) so the store stays free of provider imports (no import cycle).
- Wired at the consumer surfaces AFTER a successful mutation: the cronjob model
  tool (tools/cronjob_tools.py, create/update/remove/pause/resume) — which the
  `hermes cron` CLI also routes through — and the REST handlers
  (gateway/platforms/api_server.py, same five). Built-in's no-op default = zero
  behavior change on the default path. Sleeping-agent direct jobs.json writes
  (no tool/CLI/REST) are covered by reconcile-on-wake in start().

F.2 — config: cron.chronos.{portal_url,callback_url,expected_audience,
nas_jwks_url}. All non-secret; the agent holds no scheduler creds and the
outbound provision call reuses the existing Nous token (no token key). Additive
deep-merge key, no version literal.

F.3 — docs:
- docs/chronos-managed-cron-contract.md: authoritative agent↔NAS wire contract
  (the three agent-cron endpoints + inbound /api/cron/fire + the 3-hop trust
  model + at-most-once/re-arm semantics). This is what the NAS-side agent builds
  against.
- cron-internals.md: "Managed cron (Chronos) for scale-to-zero" section.
- cli-commands.md: cron.provider accepts chronos + the cron.chronos.* keys.
- User docs name no scheduler vendor (QStash is a NAS-internal detail).

INVARIANT re-verified: zero qstash/upstash hits across plugins/cron, gateway,
hermes_cli, tools, website/docs (the one remaining repo hit is an unrelated
Context7 MCP comment in tools/mcp_tool.py).

Tests: test_jobs_changed_notify (5) — notify calls provider hook, swallows
errors, built-in harmless, tool create/remove notify. Full cron + chronos +
webhook + config + api_server_jobs suites green (504 in the cron+chronos+webhook
run).

tools/cronjob_tools.py

@@ -33,6 +33,16 @@ from cron.jobs import (
 )
 
 
+def _notify_provider_jobs_changed_safe() -> None:
+    """Tell the active cron scheduler provider the job set changed (no-op for
+    the built-in). Best-effort — never lets a provider error break the tool."""
+    try:
+        from cron.scheduler import _notify_provider_jobs_changed
+        _notify_provider_jobs_changed()
+    except Exception:
+        pass
+
+
 # ---------------------------------------------------------------------------
 # Cron prompt scanning
 # ---------------------------------------------------------------------------
@@ -549,6 +559,7 @@ def cronjob(
                 workdir=_normalize_optional_job_value(workdir),
                 no_agent=_no_agent,
             )
+            _notify_provider_jobs_changed_safe()
             return json.dumps(
                 {
                     "success": True,
@@ -604,6 +615,7 @@ def cronjob(
             removed = remove_job(job_id)
             if not removed:
                 return tool_error(f"Failed to remove job '{job_id}'", success=False)
+            _notify_provider_jobs_changed_safe()
             return json.dumps(
                 {
                     "success": True,
@@ -619,10 +631,12 @@ def cronjob(
 
         if normalized == "pause":
             updated = pause_job(job_id, reason=reason)
+            _notify_provider_jobs_changed_safe()
             return json.dumps({"success": True, "job": _format_job(updated)}, indent=2)
 
         if normalized == "resume":
             updated = resume_job(job_id)
+            _notify_provider_jobs_changed_safe()
             return json.dumps({"success": True, "job": _format_job(updated)}, indent=2)
 
         if normalized in {"run", "run_now", "trigger"}:
@@ -711,6 +725,7 @@ def cronjob(
             if not updates:
                 return tool_error("No updates provided.", success=False)
             updated = update_job(job_id, updates)
+            _notify_provider_jobs_changed_safe()
             return json.dumps({"success": True, "job": _format_job(updated)}, indent=2)
 
         return tool_error(f"Unknown cron action '{action}'", success=False)