fix(slack): keep thread session fork/history context after first turn (#23843)

* Slack thread sessions: keep forking and history context after first turn

* Update CHANGELOG.md

src/config/sessions/types.ts

@@ -35,6 +35,8 @@ export type SessionEntry = {
   sessionFile?: string;
   /** Parent session key that spawned this session (used for sandbox session-tool scoping). */
   spawnedBy?: string;
+  /** True after a thread/topic session has been forked from its parent transcript once. */
+  forkedFromParent?: boolean;
   /** Subagent spawn depth (0 = main, 1 = sub-agent, 2 = sub-sub-agent). */
   spawnDepth?: number;
   systemSent?: boolean;