[benbrandt]

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

Shape

// session/set_title
SetSessionTitleRequest  { sessionId: SessionId, title: string, _meta? }
SetSessionTitleResponse { title: string, _meta? }   // the title the Agent stored, possibly normalized

• Required and strict: both title fields. A missing, null or non-string title is rejected, because the Agent stores the request title and the Client displays the response title.
• Capability: sessionCapabilities.title in v1 and session.title in v2, as a SessionTitleCapabilities object where {} means supported. Agents still report titles through session/list and session_info_update without it.

Capability name change from the RFD

The RFD proposed setTitle. Every other session capability is either a method verb (delete, fork, resume) or a feature name (mcp, systemPrompt, additionalDirectories), so setTitle would have been the only verb-plus-noun key. title follows the feature-name style, and later title features can add fields to it, such as the initial title on session/new proposed in #1978. I updated the RFD's capability section and revision history to match.

Docs

• v2 overview: a row in the unstable features table.
• initialization pages (v1 and v2): a title field.
• error pages (v1 and v2): unknown session gets -32002 (the session doesn't have to be active), and a rejected title gets -32602.
• Generated: unstable JSON schemas and draft schema.mdx pages.