Add dedicated-outbound iMessage group chats

[dimavrem22]

Summary

Ports the Hermes bridge's iMessage group-chat flow (inkbox-ai/hermes-agent-plugin#76) so the two behave the same on the wire.

Inbound — a group is one shared context

A group conversation is a single context that everyone in it shares, so the conversation is the chat, not the sender. Every participant's message lands in the same session, which is what lets the agent follow a thread rather than seeing N disconnected 1:1s. One-to-one keeps its per-contact chat exactly as before.

The prompt gains a group marker and the response policy:

[inkbox:group_imessage conversation_id=… from=… participants=…,… reply_mode=conversation_id | contact=…]
Group iMessage response policy: you receive every message in this group so you can track context.
Reply only when the latest message clearly addresses this Inkbox agent, asks it to act, or a
visible answer would be expected from the agent.
Treat ordinary group chatter as context only.
If no visible reply is warranted, return exactly [SILENT].

That is the same policy this bridge already applies to group SMS — _group_imessage_prompt deliberately mirrors _group_sms_prompt rather than importing Hermes's internals, so it reads like the rest of the file.

Group flags and participants come from the event when present, falling back to a conversation lookup for events that omit them (mirroring _lookup_text_conversation_summary).

Outbound — starting a group

to now accepts a single E.164 recipient or a list of 2–8 to open a group. This bridge previously had no to at all — the tool was conversation_id-only on the grounds that iMessage is recipient-first. That stays true for shared and dedicated inbound lines; a dedicated outbound line is the exception, and only it may initiate.

The tool checks for that line and returns a specific error code rather than letting the send fail at the API:

imessage_group_requires_dedicated_outbound

plus validation for exactly-one-of to/conversation_id, at least one recipient, at most 8, and distinct.

Tests

6 new: group routing into the shared conversation chat with the policy and participants present; 1:1 keeping its per-contact chat with no policy injected; and the four outbound rejection paths (dedicated-line gate, both-targets, >8, duplicates).

314 pass. The 14 tests/test_sessions.py failures are pre-existing on main — a TypeError from an SDK mismatch in this environment, unrelated and confirmed against a clean checkout.

Version

0.2.6 → 0.2.7, matching the Hermes group-chat release so the fleet stays on one number.