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>

AGENTS.md

@@ -38,7 +38,7 @@ hermes-agent/
 │   │                     #   homeassistant, signal, matrix, mattermost, email, sms,
 │   │                     #   dingtalk, wecom, weixin, feishu, qqbot, bluebubbles,
 │   │                     #   webhook, api_server, ...). See ADDING_A_PLATFORM.md.
-│   └── builtin_hooks/    # Always-registered gateway hooks (boot-md, ...)
+│   └── builtin_hooks/    # Extension point for always-registered gateway hooks (none shipped)
 ├── plugins/              # Plugin system (see "Plugins" section below)
 │   ├── memory/           # Memory-provider plugins (honcho, mem0, supermemory, ...)
 │   ├── context_engine/   # Context-engine plugins