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-proxy.ts

@@ -36,6 +36,8 @@ import type {} from '@deepseek-ai/dsh-session-projection-cache'
 import { GoalError } from '@deepseek-ai/dsh-goal'
 import type { GoalRef as CoreGoalRef } from '@deepseek-ai/dsh-goal'
 // Type-only edges: resolve `ctx.get('commands')`, the `commands/change` event, and `ctx.get('skills')`.
+// Type-only edge: resolves `ctx.get('sessionTitle')` for the rename impl.
+import type {} from '@deepseek-ai/dsh-session-title'
 import type {} from '@deepseek-ai/dsh-commands'
 import type {} from '@deepseek-ai/dsh-skill'
 import type { CallId } from '@deepseek-ai/dsh-llm/brand'
@@ -1049,6 +1051,26 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
         }
       },
 
+      async rename(request) {
+        const { sessionId, title } = request.payload
+        const found = await agentFor(sessionId)
+        if ('error' in found) return err(request, found.error)
+        const titles = ctx.get('sessionTitle')
+        if (titles === undefined) {
+          return err(request, { code: 'internal', message: 'session-title service is absent: this deployment does not mount @deepseek-ai/dsh-session-title in its composition (cordis.yml or explicit assembly)', details: {} })
+        }
+        try {
+          const accepted = titles.rename(found.agent.session, title)
+          return ok(request, { title: accepted.title, seq: accepted.eventSeq })
+        } catch (error: unknown) {
+          return err(request, {
+            code: 'title-invalid',
+            message: `rename rejected for session "${sessionId}": ${String(error)}`,
+            details: { sessionId },
+          })
+        }
+      },
+
       async prompt(request) {
         const { sessionId, mode, content } = request.payload
         const found = await agentFor(sessionId)