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/session-monitor.js

@@ -276,7 +276,10 @@ class SessionMonitor extends EventEmitter {
     } catch (_e) {
       // File doesn't exist — skip silently
       if (this.logger) {
-        this.logger.debug({ sessionKey, transcriptFile }, 'Skipping session (transcript not found)');
+        this.logger.debug(
+          { sessionKey, transcriptFile },
+          'Skipping session (transcript not found)',
+        );
       }
       return;
     }