## Why
Local message boards were scoped to an agent tree. Independently created Codex runtimes need a way to share a board while receiving different access permissions.
## What changed
- Add `LocalBoard` and `LocalBoardConnection` so hosts can supply read-only or read-write access through `thread_extension_init`, including for isolated, ephemeral runtimes.
- Allow read-only connections to read and manage their own subscriptions; reject posting, replying, channel creation, and changes to another agent's subscriptions. Expose effective permissions in channel discovery.
- Use unique runtime paths and require matching board IDs and canonical database paths when resolving agents or delivering notifications.
- Document explicit extension installation and connection setup for new runtimes, cold resumes, and forks.
## Testing
Add a scenario and context snapshot covering an independent writer and isolated Guardian sharing a board through a symlink alias, read-only permission denials, subscription and explicit notifications, and rejection of notifications to an unconnected runtime before a post is written.
GitOrigin-RevId: f92410c3a5c6a46920c3e0e71849c85dc83d9c31
jif
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