feat(session-title): user rename pins the title against automatic generation

sessionTitle.rename appends a user-source session/title event; onUserMessage
skips scheduling while a user title stands, and an explicit refresh is the
deliberate unpin (provider regeneration, or a re-derived fallback when no
provider is registered).

docs/core-data-structures/session-title.md

@@ -34,6 +34,10 @@ type SessionTitleSource =
     readonly provider: SessionTitleProviderId
     readonly model?: SessionTitleModelProvenance
   }
+  | {
+    /** Explicit user rename: pins the title — automatic generation stops scheduling. */
+    readonly kind: 'user'
+  }
 ```
 
 ```ts type-equiv