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.
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