feat(mcp): thread `session_id` through `tool_checkpoint` and declare it in the input schemas

[jphein]

The mempalace-side half of the #360 deferral, flagged while implementing mempalace checkpoint in #406. techempower-org/palace-daemon#240 asks for the indexed query; this is the part that has to exist first, because today there is nothing to index.

session_id is a write-only dead slot

Every reference to it in mempalace/mcp_server.py, exhaustively:

5024:    session_id: str = "",              # tool_diary_write parameter
5125:        if session_id:                 # the only write
5126:            base_metadata["session_id"] = session_id

That is the whole surface. Which means:

• Nothing can set it through MCP. Neither mempalace_diary_write nor mempalace_checkpoint declares session_id in its input_schema, so no MCP client can pass it. Only a direct Python call reaches the parameter.

• Nothing reads it. mempalace/searcher.py and mempalace/cli.py contain zero references, and no tool filters on it.

• tool_checkpoint drops it even where it could. It writes its diary entry via

  out["diary"] = tool_diary_write(
      agent_name=diary.get("agent_name", "cursor-ide"),
      entry=entry,
      topic=diary.get("topic", "session-checkpoint"),
      wing=diary.get("wing", ""),
  )

  — stopping one argument short of the session_id that tool_diary_write is already able to store. The default topic is literally "session-checkpoint", so the call site knows it is filing a session; it just has no way to say which.

The drawers tool_checkpoint files through tool_add_drawer carry no session metadata at all, so even a fully-threaded diary entry would leave the batch it summarizes unlinked.

Ask

1. Accept session_id on tool_checkpoint and pass it to tool_diary_write.
2. Attach it to the drawers filed in the same call, so a session's diary entry and its drawers can be recovered together rather than the summary alone.
3. Declare it in the mempalace_checkpoint and mempalace_diary_write input schemas, so MCP clients — the actual writers in practice — can set it.

With those, mempalace checkpoint --session <id> becomes a real flag rather than one with nothing behind it, and techempower-org/palace-daemon#240 gets a populated key to index.

Worth a maintainer decision, not an assumption

Whether session_id should be free-form or validated. Every other name-shaped field on this path goes through sanitize_name/sanitize_kg_value; session_id currently goes through neither, because nothing reachable ever populated it. If it becomes an indexed key it becomes a query surface, and an unvalidated one is worth deciding on deliberately rather than inheriting by default. Happy to implement whichever way it should go.

Also unresolved: whether existing diary drawers should be backfilled. Hook-written entries have never carried the field, so a --session query would silently return nothing for any session predating this change — arguably fine, but it should be a choice rather than a surprise.

Related

• feat(cli): add mempalace checkpoint command #360 — the CLI issue whose --session / checkpoint list shape this unblocks
• feat(cli): first-class verbs for the read-family MCP tools (#356, #358, #360, #362) #406 — implemented mempalace checkpoint over the real schema, without those flags
• feat: session-scoped checkpoint index — indexed query by session_id metadata palace-daemon#240 — the indexed-query consumer