fix: stale offset reset + incremental _knownSessions tracking

src/status-watcher.js:
- When lastOffset > fileSize (stale offset from compaction or previous session),
  reset offset to current file end rather than 0.
  Resetting to 0 caused re-parsing gigabytes of old content; resetting to fileSize
  means we only read new bytes from this point forward. This was the root cause of
  the status box receiving no updates — the offset was past EOF so every read
  returned 0 bytes silently.

src/session-monitor.js:
- _knownSessions is now maintained incrementally instead of being replaced wholesale
  at the end of every poll cycle.
- Previously: _knownSessions = currentSessions at end of _poll() meant forgetSession()
  had no effect — the next poll immediately re-added the key as 'known' without firing
  _onSessionAdded, silently swallowing the reactivation.
- Now: entries are added/updated individually, removals delete from the map directly.
  forgetSession() + clearCompleted() + pollNow() now correctly triggers reactivation.

Verified: 3 consecutive 5s tests all show plugin KV updating with lines and timestamps.

src/session-monitor.js

@@ -379,27 +379,31 @@ class SessionMonitor extends EventEmitter {
 
     // Detect new or previously-stale sessions
     for (const [sessionKey, entry] of currentSessions) {
-      if (!this._knownSessions.has(sessionKey) || this._staleSessions.has(sessionKey)) {
-        // Skip sessions in completed cooldown — only allow re-detection once the
-        // transcript has gone stale (file not modified in >5 min). This prevents
-        // the reactivation loop caused by the gateway appending bookkeeping events
-        // to the JSONL after a session's agent turn finishes.
+      const isKnown = this._knownSessions.has(sessionKey);
+      const isStale = this._staleSessions.has(sessionKey);
+
+      if (!isKnown || isStale) {
+        // Skip sessions in completed cooldown — only allow re-detection when:
+        // (a) completedSessions cooldown was explicitly cleared (ghost watch / clearCompleted), OR
+        // (b) sessions.json shows a newer updatedAt than when we marked complete.
         if (this._completedSessions.has(sessionKey)) {
           const completedAt = this._completedSessions.get(sessionKey);
-          // Re-activate if sessions.json shows a newer updatedAt than when we
-          // marked the session complete. This is the infrastructure-level signal
-          // that the gateway started a new turn — no AI involvement, pure timestamp
-          // comparison between two on-disk data sources.
           const sessionUpdatedAt = entry.updatedAt || 0;
           if (sessionUpdatedAt > completedAt) {
             // New turn started after completion — reactivate
             this._completedSessions.delete(sessionKey);
           } else {
-            // No new turn yet — suppress re-detection
+            // No new turn yet — add to knownSessions silently so we don't re-emit
+            // on every poll cycle, but don't fire _onSessionAdded.
+            if (!isKnown) this._knownSessions.set(sessionKey, entry);
             continue;
           }
         }
+        this._knownSessions.set(sessionKey, entry);
         this._onSessionAdded(entry);
+      } else {
+        // Update the stored entry with latest data (e.g. updatedAt changes)
+        this._knownSessions.set(sessionKey, entry);
       }
     }
 
@@ -407,6 +411,7 @@ class SessionMonitor extends EventEmitter {
     for (const [sessionKey] of this._knownSessions) {
       if (!currentSessions.has(sessionKey)) {
         this._onSessionRemoved(sessionKey);
+        this._knownSessions.delete(sessionKey);
         this._staleSessions.delete(sessionKey);
         this._completedSessions.delete(sessionKey);
       }
@@ -419,7 +424,7 @@ class SessionMonitor extends EventEmitter {
       }
     }
 
-    this._knownSessions = currentSessions;
+    // Note: _knownSessions is now maintained incrementally above, not replaced wholesale.
   }
 
   /**

src/status-watcher.js

@@ -258,16 +258,18 @@ class StatusWatcher extends EventEmitter {
       const stat = fs.fstatSync(fd);
       const fileSize = stat.size;
 
-      // Detect file truncation (compaction)
+      // Detect file truncation (compaction) or stale offset from a previous session.
+      // In both cases reset to current file end — we don't want to re-parse old content,
+      // only new bytes written from this point forward.
       if (fileSize < state.lastOffset) {
         if (this.logger) {
           this.logger.warn(
             { sessionKey, fileSize, lastOffset: state.lastOffset },
-            'Transcript truncated (compaction detected) — resetting offset',
+            'Transcript offset past file end (compaction/stale) — resetting to file end',
           );
         }
-        state.lastOffset = 0;
-        state.lines = ['[session compacted - continuing]'];
+        state.lastOffset = fileSize;
+        state.lines = [];
         state.pendingToolCalls = 0;
       }