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.

cron/scheduler.py

@@ -44,7 +44,7 @@ logger = logging.getLogger(__name__)
 _KNOWN_DELIVERY_PLATFORMS = frozenset({
     "telegram", "discord", "slack", "whatsapp", "signal",
     "matrix", "mattermost", "homeassistant", "dingtalk", "feishu",
-    "wecom", "sms", "email", "webhook",
+    "wecom", "sms", "email", "webhook", "bluebubbles",
 })
 
 from cron.jobs import get_due_jobs, mark_job_run, save_job_output, advance_next_run
@@ -91,7 +91,7 @@ def _resolve_delivery_target(job: dict) -> Optional[dict]:
             }
         # Origin missing (e.g. job created via API/script) — try each
         # platform's home channel as a fallback instead of silently dropping.
-        for platform_name in ("matrix", "telegram", "discord", "slack"):
+        for platform_name in ("matrix", "telegram", "discord", "slack", "bluebubbles"):
             chat_id = os.getenv(f"{platform_name.upper()}_HOME_CHANNEL", "")
             if chat_id:
                 logger.info(
@@ -236,6 +236,7 @@ def _deliver_result(job: dict, content: str, adapters=None, loop=None) -> Option
         "wecom": Platform.WECOM,
         "email": Platform.EMAIL,
         "sms": Platform.SMS,
+        "bluebubbles": Platform.BLUEBUBBLES,
     }
     platform = platform_map.get(platform_name.lower())
     if not platform: