feat: Phase 3 — sub-agent detection, nested status, cascade completion

Phase 3 (Sub-Agent Support):
- session-monitor.js: sub-agents always passed through (inherit parent channel)
- watcher-manager.js enhancements:
  - Pending sub-agent queue: child sessions that arrive before parent are queued
    and processed when parent is registered (no dropped sub-agents)
  - linkSubAgent(): extracted helper for clean parent-child linking
  - Cascade completion: parent stays active until all children complete
  - Sub-agents embedded in parent status post (no separate top-level post)
- status-formatter.js: recursive nested rendering at configurable depth

Integration tests - test/integration/sub-agent.test.js (9 tests):
  3.1 Sub-agent detection via spawnedBy (monitor level)
  3.2 Nested status rendering (depth indentation, multiple children, deep nesting)
  3.3 Cascade completion (pending tool call tracking across sessions)
  3.4 Sub-agent JSONL parsing (usage events, error tool results)

All 95 tests pass (59 unit + 36 integration). make check clean.

src/session-monitor.js

@@ -226,11 +226,14 @@ class SessionMonitor extends EventEmitter {
 
     const transcriptFile = this._transcriptPath(agentId, sessionId);
 
+    // Sub-agents always pass through — they inherit parent channel via watcher-manager
+    const isSubAgent = !!spawnedBy;
+
     // Resolve channel ID from session key
     let channelId = SessionMonitor.parseChannelId(sessionKey);
 
     // Fall back to default channel for non-MM sessions
-    if (!channelId && !SessionMonitor.isMattermostSession(sessionKey)) {
+    if (!channelId && !isSubAgent && !SessionMonitor.isMattermostSession(sessionKey)) {
       channelId = this.defaultChannel;
       if (!channelId) {
         if (this.logger) {