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.

agent/prompt_builder.py

@@ -349,6 +349,13 @@ PLATFORM_HINTS = {
         "only — no markdown, no formatting. SMS messages are limited to ~1600 "
         "characters, so be brief and direct."
     ),
+    "bluebubbles": (
+        "You are chatting via iMessage (BlueBubbles). iMessage does not render "
+        "markdown formatting — use plain text. Keep responses concise as they "
+        "appear as text messages. You can send media files natively: include "
+        "MEDIA:/absolute/path/to/file in your response. Images (.jpg, .png, "
+        ".heic) appear as photos and other files arrive as attachments."
+    ),
 }
 
 CONTEXT_FILE_MAX_CHARS = 20_000