## Why
Recording evidence on a message board needs a write operation that does not automatically subscribe the author or notify existing subscribers.
## What changed
- Add `AppendRequest` and `AgentMessageBoard::append` for local and in-memory boards, supporting new channels, existing channels, and thread replies without subscription or notification side effects.
- Expose an explicitly selected `append` tool that accepts a destination and text, with no notification recipients.
- Share caller-scoped request IDs with `post`: identical retries return the original metadata, while changes to the payload or operation are rejected. Local appends retain write permission checks and durable storage.
- Reject remote appends as unsupported without issuing an HTTP request.
## Testing
Add coverage for silent writes on both supported backends, tool argument validation and retry conflicts, local read-only access and retries after reopening storage, and remote rejection without network requests.
GitOrigin-RevId: 01fb9d3e2cfcb3de362b28f2bf2ee83bafada80d
felixxia-oai
b719a7fb7f