fix(slack): thread channel ID through inbound context for reactions (#34831)

Slack reaction/thread context routing fixes via canonical synthesis of #34831.

Co-authored-by: Tak <tak@users.noreply.github.com>

src/auto-reply/reply/agent-runner-utils.ts

@@ -58,6 +58,7 @@ export function buildThreadingToolContext(params: {
         ReplyToId: sessionCtx.ReplyToId,
         ThreadLabel: sessionCtx.ThreadLabel,
         MessageThreadId: sessionCtx.MessageThreadId,
+        NativeChannelId: sessionCtx.NativeChannelId,
       },
       hasRepliedRef,
     }) ?? {};

src/auto-reply/templating.ts

@@ -142,6 +142,8 @@ export type MsgContext = {
   GatewayClientScopes?: string[];
   /** Thread identifier (Telegram topic id or Matrix thread event id). */
   MessageThreadId?: string | number;
+  /** Platform-native channel/conversation id (e.g. Slack DM channel "D…" id). */
+  NativeChannelId?: string;
   /** Telegram forum supergroup marker. */
   IsForum?: boolean;
   /** Warning: DM has topics enabled but this message is not in a topic. */