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/tests/client-handler.spec.ts

@@ -46,6 +46,7 @@ function scriptedApi(overrides: {
       selectModel: r => ok(r, {
         selected: { provider: r.payload.provider, model: r.payload.model },
       }),
+      rename: r => ok(r, { title: 'renamed', seq: 0 }),
       prompt: r => ok(r, { accepted: true as const }),
       cancel: r => ok(r, { accepted: true as const }),
       ...overrides.sessions,