feat(gateway): add BlueBubbles iMessage platform adapter (#6437)

Adds Apple iMessage as a gateway platform via BlueBubbles macOS server.

Architecture:
- Webhook-based inbound (event-driven, no polling/dedup needed)
- Email/phone → chat GUID resolution for user-friendly addressing
- Private API safety (checks helper_connected before tapback/typing)
- Inbound attachment downloading (images, audio, documents cached locally)
- Markdown stripping for clean iMessage delivery
- Smart progress suppression for platforms without message editing

Based on PR #5869 by @benjaminsehl (webhook architecture, GUID resolution,
Private API safety, progress suppression) with inbound attachment downloading
from PR #4588 by @1960697431 (attachment cache routing).

Integration points: Platform enum, env config, adapter factory, auth maps,
cron delivery, send_message routing, channel directory, platform hints,
toolset definition, setup wizard, status display.

27 tests covering config, adapter, webhook parsing, GUID resolution,
attachment download routing, toolset consistency, and prompt hints.

hermes_cli/config.py

@@ -39,6 +39,7 @@ _EXTRA_ENV_KEYS = frozenset({
     "DINGTALK_CLIENT_ID", "DINGTALK_CLIENT_SECRET",
     "FEISHU_APP_ID", "FEISHU_APP_SECRET", "FEISHU_ENCRYPT_KEY", "FEISHU_VERIFICATION_TOKEN",
     "WECOM_BOT_ID", "WECOM_SECRET",
+    "BLUEBUBBLES_SERVER_URL", "BLUEBUBBLES_PASSWORD",
     "TERMINAL_ENV", "TERMINAL_SSH_KEY", "TERMINAL_SSH_PORT",
     "WHATSAPP_MODE", "WHATSAPP_ENABLED",
     "MATTERMOST_HOME_CHANNEL", "MATTERMOST_REPLY_MODE",