fix: remove dead delete+recreate and pin code, add poll fallback test

Phase 1 cleanup:
- Remove deletePost() method (dead code, replaced by PUT in-place updates)
- Remove _postInfo Map tracking (no longer needed)
- Remove pin/unpin API calls from watcher-manager.js (incompatible with PUT updates)
- Add JSDoc note on (edited) label limitation in _flushUpdate()
- Add integration test: test/integration/poll-fallback.test.js
- Fix addSession() lastOffset===0 falsy bug (0 was treated as 'no offset')
- Fix pre-existing test failures: add lastOffset:0 where tests expect backlog reads
- Fix pre-existing session-monitor test: create stub transcript files
- Fix pre-existing status-formatter test: update indent check for blockquote format
- Format plugin/ files with Prettier (pre-existing formatting drift)

src/status-formatter.js

@@ -70,7 +70,12 @@ function format(sessionState, opts = {}) {
   // never collapses like code blocks do, supports inline markdown
   var body = lines.join('\n');
   if (depth === 0) {
-    body = body.split('\n').map(function (l) { return '> ' + l; }).join('\n');
+    body = body
+      .split('\n')
+      .map(function (l) {
+        return '> ' + l;
+      })
+      .join('\n');
   }
   return body;
 }