fix(plugins): register dynamically-loaded modules in sys.modules before exec

Dashboard plugin API routes (web_server._mount_plugin_api_routes) and
gateway event hooks (gateway.hooks.HookRegistry.discover_and_load) both
loaded Python files via importlib.util.spec_from_file_location +
exec_module without registering the resulting module in sys.modules.

That breaks any plugin or hook handler that uses `from __future__ import
annotations` together with a Pydantic BaseModel / dataclass / anything
that introspects `__module__`: at first request Pydantic tries to
resolve string-form type hints against the defining module's namespace,
can't find it by name, and raises:

  PydanticUserError: TypeAdapter[...] is not fully defined;
  you should define ... and all referenced types,
  then call `.rebuild()` on the instance.

This is what broke the kanban dashboard's 'triage' button — POST
/api/plugins/kanban/tasks validated against CreateTaskBody (a Pydantic
model in a file using `from __future__ import annotations`) and
returned 500 on every click.

The fix, applied symmetrically to both loaders:

  1. Compute module_name once.
  2. Register the module in sys.modules BEFORE exec_module.
  3. On exec_module failure, pop the half-initialized stub so subsequent
     reloads don't pick up broken state.

GETs were unaffected because they don't build a body TypeAdapter, which
is why this only surfaced when users started POSTing.

gateway/hooks.py

@@ -21,6 +21,7 @@ Errors in hooks are caught and logged but never block the main pipeline.
 
 import asyncio
 import importlib.util
+import sys
 from typing import Any, Callable, Dict, List, Optional
 
 import yaml
@@ -97,16 +98,28 @@ class HookRegistry:
                     print(f"[hooks] Skipping {hook_name}: no events declared", flush=True)
                     continue
 
-                # Dynamically load the handler module
+                # Dynamically load the handler module.
+                # Register in sys.modules BEFORE exec_module so Pydantic /
+                # dataclasses / typing introspection can resolve forward
+                # references (triggered by `from __future__ import annotations`
+                # in the handler). Without this, a handler that declares a
+                # Pydantic BaseModel for webhook/event payloads fails at first
+                # dispatch with "TypeAdapter ... is not fully defined".
+                module_name = f"hermes_hook_{hook_name}"
                 spec = importlib.util.spec_from_file_location(
-                    f"hermes_hook_{hook_name}", handler_path
+                    module_name, handler_path
                 )
                 if spec is None or spec.loader is None:
                     print(f"[hooks] Skipping {hook_name}: could not load handler.py", flush=True)
                     continue
 
                 module = importlib.util.module_from_spec(spec)
-                spec.loader.exec_module(module)
+                sys.modules[module_name] = module
+                try:
+                    spec.loader.exec_module(module)
+                except Exception:
+                    sys.modules.pop(module_name, None)
+                    raise
 
                 handle_fn = getattr(module, "handle", None)
                 if handle_fn is None: