feat: built-in boot-md hook — run BOOT.md on gateway startup (#3733)

The gateway now ships with a built-in boot-md hook that checks for
~/.hermes/BOOT.md on every startup. If the file exists, the agent
executes its instructions in a background thread. No installation
or configuration needed — just create the file.

No BOOT.md = zero overhead (the hook silently returns).

Implementation:
- gateway/builtin_hooks/boot_md.py: handler with boot prompt,
  background thread, [SILENT] suppression, error handling
- gateway/hooks.py: _register_builtin_hooks() called at the start
  of discover_and_load() to wire in built-in hooks
- Docs updated: hooks page documents BOOT.md as a built-in feature

gateway/hooks.py

@@ -51,14 +51,33 @@ class HookRegistry:
         """Return metadata about all loaded hooks."""
         return list(self._loaded_hooks)
 
+    def _register_builtin_hooks(self) -> None:
+        """Register built-in hooks that are always active."""
+        try:
+            from gateway.builtin_hooks.boot_md import handle as boot_md_handle
+
+            self._handlers.setdefault("gateway:startup", []).append(boot_md_handle)
+            self._loaded_hooks.append({
+                "name": "boot-md",
+                "description": "Run ~/.hermes/BOOT.md on gateway startup",
+                "events": ["gateway:startup"],
+                "path": "(builtin)",
+            })
+        except Exception as e:
+            print(f"[hooks] Could not load built-in boot-md hook: {e}", flush=True)
+
     def discover_and_load(self) -> None:
         """
         Scan the hooks directory for hook directories and load their handlers.
 
+        Also registers built-in hooks that are always active.
+
         Each hook directory must contain:
           - HOOK.yaml with at least 'name' and 'events' keys
           - handler.py with a top-level 'handle' function (sync or async)
         """
+        self._register_builtin_hooks()
+
         if not HOOKS_DIR.exists():
             return