refactor(gateway): extract kanban watcher loops into GatewayKanbanWatchersMixin (god-file Phase 3)

gateway/run.py is the largest god file (20k LOC, GatewayRunner with 220
methods). This lifts the cohesive kanban-watcher cluster — _kanban_notifier_watcher,
_kanban_dispatcher_watcher, _kanban_advance/unsub/rewind, _deliver_kanban_artifacts
(~1,035 LOC, 6 methods) — into gateway/kanban_watchers.py as a mixin that
GatewayRunner inherits.

Mixin (not free functions) because the methods use only self state: inheriting
keeps every self._kanban_* call site working unchanged via the MRO, making this
a behavior-neutral move. The methods' lazy imports (_kb, _decomp, _load_config,
Platform) travel with them; the mixin needs only stdlib + a matching
logging.getLogger('gateway.run').

run.py 20187 -> 19157 LOC; GatewayRunner direct methods 220 -> 214.

Behavior-neutral: gateway test suite 6582 passed / 0 failed; start() still wires
both watchers via self._kanban_*; MRO resolves all 6 to the mixin. One test
(corrupt-board quarantine retry) keyed its time-travel mock on the caller's
filename being gateway/run.py — updated to also accept gateway/kanban_watchers.py.

Establishes the mixin-extraction pattern for further GatewayRunner decomposition
(the 2406-LOC _run_agent and 1164-LOC _handle_message remain, but their callback
closures need a context-object redesign — deferred).

tests/hermes_cli/test_kanban_core_functionality.py

@@ -3754,11 +3754,15 @@ def test_gateway_dispatcher_retries_corrupt_board_after_quarantine(
         caller = inspect.currentframe().f_back  # type: ignore[union-attr]
         code = caller.f_code if caller is not None else None
         filename = code.co_filename if code is not None else ""
-        if filename.endswith("gateway/run.py"):
+        # The kanban dispatcher/notifier watcher loops were extracted from
+        # gateway/run.py into gateway/kanban_watchers.py (god-file Phase 3),
+        # so accept either filename for the time-travel mock.
+        if filename.endswith("gateway/run.py") or filename.endswith("gateway/kanban_watchers.py"):
             return next(time_values, 1301.0)
         return real_monotonic()
 
     monkeypatch.setattr("gateway.run.time.monotonic", _monotonic_for_gateway_dispatcher)
+    monkeypatch.setattr("gateway.kanban_watchers.time.monotonic", _monotonic_for_gateway_dispatcher)
 
     calls = {"tick": 0}