remove: BOOT.md built-in hook (#17093)

BOOT.md was merged in PR #3733 before the feature was ready — the
built-in hook spawned a bare AIAgent() with no model/runtime kwargs,
which immediately 401s on any provider with a custom endpoint. Three
separate community PRs (#5240, #12514, #14992) tried to paper over it.

Remove the BOOT.md hook entirely and its user-facing docs/tips. Keep
the gateway/builtin_hooks/ package and the HookRegistry._register_builtin_hooks()
hook-point intact as the extension surface for future always-on
gateway hooks.

Closes #5239.

Co-authored-by: teknium1 <teknium@users.noreply.github.com>

website/docs/developer-guide/architecture.md

@@ -116,7 +116,7 @@ hermes-agent/
 │   ├── hooks.py              # Hook discovery and lifecycle events
 │   ├── mirror.py             # Cross-session message mirroring
 │   ├── status.py             # Token locks, profile-scoped process tracking
-│   ├── builtin_hooks/        # Always-registered hooks
+│   ├── builtin_hooks/        # Extension point for always-registered hooks (none shipped)
 │   └── platforms/            # 18 adapters: telegram, discord, slack, whatsapp,
 │                             #   signal, matrix, mattermost, email, sms,
 │                             #   dingtalk, feishu, wecom, wecom_callback, weixin,