Add main-loop enable/disable per-agent toggle

- self_main_loop.py: 'enabled' map in config (default all True);
  do_check skips disabled agents (marks disabled:true in results);
  new enable/disable actions with optional --agent.
- box-ctl.py: main-loop enable|disable [--agent <name>] actions,
  USAGE updated.

Check skips disabled agents so the loop can be toggled per
Chromebox without stopping the systemd timer.

Session: sidechat/chromebox-ops

bin/box-ctl.py

@@ -650,18 +650,25 @@ def act_notify(agent, message, sidechat=None, allow_main_chat=False):
     out(True, agent=agent, target=target, sent=True)
 
 
-def act_main_loop(sub):
+def act_main_loop(sub, agent=None):
     """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.
+    check:   one loop iteration -> JSON {ok, prompted, errors, new_total, agents}
+    status:  watermark / last run / prompt-sidechat config, no chat reads.
+    enable:  enable the loop (per-agent or all); check skips disabled agents.
+    disable: disable the loop (per-agent or all).
     """
-    if sub not in ("check", "status"):
-        fail("BAD_NAME", "usage: main-loop check|status")
-    audit("main-loop-" + sub)
+    if sub not in ("check", "status", "enable", "disable"):
+        fail("BAD_NAME", "usage: main-loop check|status|enable|disable [--agent <name>]")
+    if agent is not None and agent not in VALID_AGENTS:
+        fail("BAD_NAME", f"agent must be one of {sorted(VALID_AGENTS)}")
+    audit("main-loop-" + sub + (f"-{agent}" if agent else ""))
     script = BIN / "self_main_loop.py"
+    cmd = [sys.executable, str(script), sub]
+    if agent:
+        cmd += ["--agent", agent]
     try:
-        r = subprocess.run([sys.executable, str(script), sub],
+        r = subprocess.run(cmd,
                            capture_output=True, text=True, timeout=600)
     except subprocess.TimeoutExpired:
         fail("LOOP_TIMEOUT", "self_main_loop.py timed out")
@@ -1171,7 +1178,8 @@ 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
+  main-loop check|status|enable|disable [--agent <name>]
+                        main-chat self-monitor; enable/disable per agent or all
 
 timer actions:
   timer-list
@@ -1415,9 +1423,20 @@ def main(argv):
             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])
+        if not rest or rest[0] not in ("check", "status", "enable", "disable"):
+            fail("BAD_NAME", "usage: main-loop check|status|enable|disable [--agent <name>]")
+        sub = rest[0]
+        agent = None
+        args = rest[1:]
+        if "--agent" in args:
+            i = args.index("--agent")
+            if i + 1 >= len(args):
+                fail("BAD_ARGS", "usage: main-loop %s [--agent <name>]" % sub)
+            agent = args[i + 1]
+            args = args[:i] + args[i + 2:]
+        if args:
+            fail("BAD_ARGS", "usage: main-loop %s [--agent <name>]" % sub)
+        act_main_loop(sub, agent)
     elif action == "dm-log":
         limit = 50
         if rest: