feat(channel): add built-in IM adapter core

[jialudev]

• Route eligible host Codex participants through a binding-scoped built-in IM adapter backed by Agent Engine while preserving existing direct, mention, room, thread, and reply behavior.
• Render final answers, thoughts, tool activity, permissions, and structured user input through the existing message and activity APIs with stable IDs and legacy-compatible metadata.
• Materialize source-scoped attachments, serialize turns with wait admission, prevent stale turn updates, and release runtime work state across success, failure, and cancellation.