feat(plugins): add pre_gateway_dispatch hook

Introduces a new plugin hook `pre_gateway_dispatch` fired once per
incoming MessageEvent in `_handle_message`, after the internal-event
guard but before the auth / pairing chain. Plugins may return a dict
to influence flow:

    {"action": "skip",    "reason": "..."}  -> drop (no reply)
    {"action": "rewrite", "text":   "..."}  -> replace event.text
    {"action": "allow"}  /  None             -> normal dispatch

Motivation: gateway-level message-flow patterns that don't fit cleanly
into any single adapter — e.g. listen-only group-chat windows (buffer
ambient messages, collapse on @mention), or human-handover silent
ingest (record messages while an owner handles the chat manually).
Today these require forking core; with this hook they can live in a
single profile-agnostic plugin.

Hook runs BEFORE auth so plugins can handle unauthorized senders
(e.g. customer-service handover ingest) without triggering the
pairing-code flow. Exceptions in plugin callbacks are caught and
logged; the first non-None action dict wins, remaining results are
ignored.

Includes:
- `VALID_HOOKS` entry + inline doc in `hermes_cli/plugins.py`
- Invocation block in `gateway/run.py::_handle_message`
- 5 new tests in `tests/gateway/test_pre_gateway_dispatch.py`
  (skip, rewrite, allow, exception safety, internal-event bypass)
- 2 additional tests in `tests/hermes_cli/test_plugins.py`
- Table entry in `website/docs/user-guide/features/plugins.md`

Made-with: Cursor

hermes_cli/plugins.py

@@ -71,6 +71,14 @@ VALID_HOOKS: Set[str] = {
     "on_session_finalize",
     "on_session_reset",
     "subagent_stop",
+    # Gateway pre-dispatch hook. Fired once per incoming MessageEvent
+    # after core auth/internal-event guards but before command handling
+    # and agent dispatch. Plugins may return a dict to influence flow:
+    #   {"action": "skip",    "reason": "..."}  -> drop message (no reply)
+    #   {"action": "rewrite", "text": "..."}    -> replace event.text, continue
+    #   {"action": "allow"}  /  None             -> normal dispatch
+    # Kwargs: event: MessageEvent, gateway: HermesGateway, session_store.
+    "pre_gateway_dispatch",
 }
 
 ENTRY_POINTS_GROUP = "hermes_agent.plugins"