Add self_main_loop.py main-chat self-monitor + box main-loop action

Reads each fleet agent's muse.ai Main chat on a 5-min systemd timer
(self-main-loop.timer). On new messages since the per-agent watermark,
posts a concise digest to that agent's prompting sidechat (dm.py, opm
as neutral sender - mirrors box notify), prompting the operator to
check main chat via DM/box. Escapes the main-chat-goes-unread failure
mode. No backfill on first run; silent when nothing new; read/send
failures logged without killing the timer; flock overlap guard.

box-ctl.py: main-loop check | status (fleet section).

Session: sidechat/chromebox-ops

bin/box-ctl.py

@@ -650,6 +650,30 @@ def act_notify(agent, message, sidechat=None, allow_main_chat=False):
     out(True, agent=agent, target=target, sent=True)
 
 
+def act_main_loop(sub):
+    """Run the main-chat self-monitor loop (self_main_loop.py).
+
+    check:  one loop iteration -> JSON {ok, prompted, errors, new_total, agents}
+    status: watermark / last run / prompt-sidechat config, no chat reads.
+    """
+    if sub not in ("check", "status"):
+        fail("BAD_NAME", "usage: main-loop check|status")
+    audit("main-loop-" + sub)
+    script = BIN / "self_main_loop.py"
+    try:
+        r = subprocess.run([sys.executable, str(script), sub],
+                           capture_output=True, text=True, timeout=600)
+    except subprocess.TimeoutExpired:
+        fail("LOOP_TIMEOUT", "self_main_loop.py timed out")
+    try:
+        data = json.loads(r.stdout.strip())
+    except Exception:
+        fail("LOOP_ERROR", "self_main_loop.py returned non-JSON",
+             {"stdout": (r.stdout or "")[-300:], "stderr": (r.stderr or "")[-300:]})
+    ok = bool(data.pop("ok", False))
+    out(ok, **data)
+
+
 def act_watchdog_alerts():
     """Check for new failed browser relaunches since the watermark.
 
@@ -1147,6 +1171,7 @@ fleet:
   relay-health
   cdp-latency
   identity-audit        VM identity audit drift check
+  main-loop check|status  main-chat self-monitor: prompt sidechat on new main msgs
 
 timer actions:
   timer-list
@@ -1389,6 +1414,10 @@ def main(argv):
         if rest:
             fail("BAD_ARGS", "usage: chrome-errors")
         act_chrome_errors()
+    elif action == "main-loop":
+        if len(rest) != 1 or rest[0] not in ("check", "status"):
+            fail("BAD_NAME", "usage: main-loop check|status")
+        act_main_loop(rest[0])
     elif action == "dm-log":
         limit = 50
         if rest: