Slack: implement replyToMode threading for tool path

- Add shared hasRepliedRef state between auto-reply and tool paths
- Extract buildSlackThreadingContext helper in agent-runner.ts
- Extract resolveThreadTsFromContext helper in slack-actions.ts
- Update docs with clear replyToMode table (off/first/all)
- Add tests for first mode behavior across multiple messages

src/auto-reply/types.ts

@@ -21,6 +21,8 @@ export type GetReplyOptions = {
   blockReplyTimeoutMs?: number;
   /** If provided, only load these skills for this session (empty = no skills). */
   skillFilter?: string[];
+  /** Mutable ref to track if a reply was sent (for Slack "first" threading mode). */
+  hasRepliedRef?: { value: boolean };
 };
 
 export type ReplyPayload = {