feat(apiproxy): session.rename RPC delegating to the session-title service

New unary method in RpcMethodMap with the title-invalid error code; the impl
resolves the agent (cold sessions resume first) and delegates to
ctx.sessionTitle.rename, returning the normalized title plus its event seq so
clients settle the title projection cell ahead of the push frame. session.fork
stays on the reserved-seam list.

packages/host/apiproxy/src/api/sessions.ts

@@ -208,6 +208,16 @@ export interface SessionsApi {
   }>):
   Promise<RpcResponse<{ selected: ModelTarget }>>
 
+  /**
+   * Renames a session: appends a `session/title` event with the `user`
+   * source, which pins the title against automatic regeneration. The
+   * normalized accepted title and the title event's seq return so the caller
+   * can settle its projection cell without waiting for the push frame. A
+   * title that normalizes to empty fails with `title-invalid`.
+   */
+  rename(request: RpcRequest<{ sessionId: SessionId; title: string }>):
+  Promise<RpcResponse<{ title: string; seq: number }>>
+
   /**
    * Sends a message. content is core's ContentBlock[] verbatim; mode maps 1:1 — queue→send, steer→steer.
    * A prompt whose content is exactly one text block starting with '/' is a slash command: the host