feat: built-in boot-md hook — run BOOT.md on gateway startup

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

website/docs/user-guide/features/hooks.md

@@ -88,6 +88,26 @@ Handlers registered for `command:*` fire for any `command:` event (`command:mode
 
 ### Examples
 
+#### Boot Checklist (BOOT.md) — Built-in
+
+The gateway ships with a built-in `boot-md` hook that looks for `~/.hermes/BOOT.md` on every startup. If the file exists, the agent runs its instructions in a background session. No installation needed — just create the file.
+
+**Create `~/.hermes/BOOT.md`:**
+
+```markdown
+# Startup Checklist
+
+1. Check if any cron jobs failed overnight — run `hermes cron list`
+2. Send a message to Discord #general saying "Gateway restarted, all systems go"
+3. Check if /opt/app/deploy.log has any errors from the last 24 hours
+```
+
+The agent runs these instructions in a background thread so it doesn't block gateway startup. If nothing needs attention, the agent replies with `[SILENT]` and no message is delivered.
+
+:::tip
+No BOOT.md? The hook silently skips — zero overhead. Create the file whenever you need startup automation, delete it when you don't.
+:::
+
 #### Telegram Alert on Long Tasks
 
 Send yourself a message when the agent takes more than 10 steps: