fix(agent-to-agent): route A2A replies back to originating session (#2267)

Squash merge of PR #2267 by ddaniels.

When an agent group has more than one active session, A2A replies landed
in the newest session via findSessionByAgentGroup's ORDER BY created_at
DESC. The session that asked the question never saw the answer.

Adds origin-aware return-path routing with three layers:

1. Direct return-path: if the reply has in_reply_to, look up the
   triggering inbound row's source_session_id and route there.
2. Peer-affinity fallback: find the most recent A2A inbound from this
   peer and use its source_session_id.
3. Legacy fallback: newest active session (pre-migration compat).

Container-side: MCP send_message/send_file now thread the current
batch's in_reply_to through to outbound rows via current-batch.ts.

Also flips our A2A bug-documenting test (#2332) from asserting the
broken behavior to asserting the fixed behavior.

Co-Authored-By: Doug Daniels <ddaniels888@gmail.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/session-manager.ts

@@ -210,6 +210,12 @@ export function writeSessionMessage(
      * a trigger-1 message does arrive.
      */
     trigger?: 0 | 1;
+    /**
+     * For agent-to-agent inbound: the source session id that emitted the
+     * outbound message which became this inbound row. Used as the return
+     * path so the target's reply routes back to that exact session.
+     */
+    sourceSessionId?: string | null;
   },
 ): void {
   // Extract base64 attachment data, save to inbox, replace with file paths
@@ -228,6 +234,7 @@ export function writeSessionMessage(
       processAfter: message.processAfter ?? null,
       recurrence: message.recurrence ?? null,
       trigger: message.trigger ?? 1,
+      sourceSessionId: message.sourceSessionId ?? null,
     });
   } finally {
     db.close();