[benbrandt]

Adds schema support for the session-status RFD behind a new unstable_session_status feature (included in unstable), in both v1 and v2.

Shape

// session/status
SessionStatusRequest  { sessionId: SessionId, _meta? }
SessionStatusResponse { status: SessionStatus, state?: StateUpdate /* v2 only */, _meta? }
SessionStatus = "open" | "saved" | string   // unrecognized values are treated as "saved"
// unknown session → -32002 with data.sessionId

• status: required and strict, because Clients decide whether to resume based on it. It uses the same open-enum pattern as SystemPromptMode, so custom _ values round-trip.
• v2 state: reuses StateUpdate. A missing, null or malformed value means "not reported", never idle, as the RFD specifies.
• Capability: sessionCapabilities.status (v1) / session.status (v2), as a SessionStatusCapabilities object.

Status values renamed from the RFD

The RFD proposed active / inactive. v2 already calls a session "active" on a connection from session/new, session/resume or session/fork until close or disconnect. A status of active that means "in memory for any connection" invites Clients to skip session/resume. open / saved follow the session lifecycle instead: session/close frees an open session, after which it reads as saved, and session/resume opens it again. The RFD is updated, with an FAQ entry and a revision history entry.

Docs

• v2 overview: a row in the unstable features table.
• initialization pages (v1 and v2): a status field.
• error pages (v1 and v2): an unknown session gets -32002, and a session that exists but isn't loaded is not an error.
• Generated: unstable JSON schemas and draft schema.mdx pages.

#2734 (session/set_title) touches the same spots in the docs and SessionCapabilities, so whichever merges second will need a small conflict resolved.