feat: show transcript file size in session status

Add transcript size monitoring to /status and session_status tool.
Displays file size and message count (e.g. '📄 Transcript: 1.2 MB,
627 messages'). Shows ⚠️ warning when transcript exceeds 1 MB, which
helps catch sessions approaching the compaction death spiral described
in #13624.

- getTranscriptInfo() reads JSONL file stat + line count
- Wired into both /status command and session_status tool
- 8 new tests covering file reading, formatting, and edge cases

src/auto-reply/reply/commands-status.ts

@@ -28,7 +28,7 @@ import {
   resolveUsageProviderId,
 } from "../../infra/provider-usage.js";
 import { normalizeGroupActivation } from "../group-activation.js";
-import { buildStatusMessage } from "../status.js";
+import { buildStatusMessage, getTranscriptInfo } from "../status.js";
 import { getFollowupQueueDepth, resolveQueueSettings } from "./queue.js";
 import { resolveSubagentLabel } from "./subagents-utils.js";
 
@@ -247,6 +247,13 @@ export async function buildStatusReply(params: {
     subagentsLine,
     mediaDecisions: params.mediaDecisions,
     includeTranscriptUsage: false,
+    transcriptInfo: getTranscriptInfo({
+      sessionId: sessionEntry?.sessionId,
+      sessionEntry,
+      agentId: statusAgentId,
+      sessionKey,
+      storePath,
+    }),
   });
 
   return { text: statusText };