feat(plugins): add pre_approval_request / post_approval_response hooks (#16776)

Plugins can now observe dangerous-command approval events in real time,
on both the CLI-interactive path and the async gateway path. This is the
missing hook surface external tools need to build approval notifiers
(macOS menu-bar allow/deny, Slack alerts, audit logs, etc.) without
forking Hermes or running a parallel gateway adapter.

Changes:
- hermes_cli/plugins.py: add two entries to VALID_HOOKS
- tools/approval.py: fire both hooks from check_all_command_guards --
  around prompt_dangerous_approval (CLI surface) and around the
  notify_cb + blocking event.wait loop (gateway surface)
- website/docs/user-guide/features/hooks.md: document both hooks with
  a macOS-notification example
- tests/tools/test_approval_plugin_hooks.py: 5 tests covering CLI once,
  CLI deny, plugin-crash resilience, gateway approve, gateway timeout

Hooks are observer-only: return values are ignored, so plugins cannot
veto or pre-answer an approval (use pre_tool_call for that). A crashing
plugin cannot break the approval flow -- invoke_hook swallows per-
callback errors, and the wrapper logs and swallows dispatch-layer
errors too.

Surface kwarg distinguishes "cli" from "gateway"; post hook reports
choice as one of once/session/always/deny/timeout.

hermes_cli/plugins.py

@@ -79,6 +79,20 @@ VALID_HOOKS: Set[str] = {
     #   {"action": "allow"}  /  None             -> normal dispatch
     # Kwargs: event: MessageEvent, gateway: GatewayRunner, session_store.
     "pre_gateway_dispatch",
+    # Approval lifecycle hooks. Fired by tools/approval.py when a dangerous
+    # command needs user approval -- fires BOTH for CLI-interactive prompts
+    # and for gateway/ACP approvals (Telegram, Discord, Slack, TUI, etc.).
+    # Observers only: return values are ignored. Plugins cannot veto or
+    # pre-answer an approval from these hooks (use pre_tool_call to block
+    # a tool before it reaches approval).
+    #
+    # Kwargs for pre_approval_request:
+    #   command: str, description: str, pattern_key: str, pattern_keys: list[str],
+    #   session_key: str, surface: "cli" | "gateway"
+    # Kwargs for post_approval_response: same as above plus
+    #   choice: "once" | "session" | "always" | "deny" | "timeout"
+    "pre_approval_request",
+    "post_approval_response",
 }
 
 ENTRY_POINTS_GROUP = "hermes_agent.plugins"