MCP: read every reply a session gives, exactly once #383

As an agent coordinating several sessions through agent-guild mcp, I want to read every reply each session gives, exactly once, so that I don't miss a worker's report or act on the same report twice.

Current behavior

• list_sessions says only whether a session has a reply (hasReply).
• read_reply returns the text of the answer that ended the session's last turn, with nothing that identifies which reply it is.
• A session keeps only its latest reply. A new reply replaces the previous one, and a turn that ends without an answer clears it.

So a coordinator that polls can't tell a new reply from one it has already handled. If a session finishes two turns between polls, the first reply is gone.

Acceptance criteria

• A coordinator can tell whether a session's reply is one it has already read.
• A coordinator that checks in less often than a session finishes turns can still read, in order, the replies it missed.
• Clients that show or read aloud the last answer (the page, VS Code, the apps, Apple Watch, Meta Ray-Ban Display) behave as they do today.
• docs/mcp.md (and docs/api.md, if the local API changes) describe the behavior, and tests cover it.

To decide

• How many past replies a session keeps, and what a coordinator sees when it asks for one that is no longer kept.

Starting points

• reportReply in src/manager/session.mjs
• summarize and read_reply in src/mcp/server.mjs