feat: RHS panel — persistent Agent Status sidebar

Added a Right-Hand Sidebar (RHS) panel to the Mattermost plugin that
shows live agent activity in a dedicated, always-visible panel.

- New RHSPanel component with SessionCard views per active session
- registerAppBarComponent adds 'Agent Status' icon to toolbar
- Subscribes to WebSocket updates via global listener
- Shows active sessions with live elapsed time, tool calls, token count
- Shows recent completed sessions below active ones
- Responsive CSS matching Mattermost design system

The RHS panel solves the scroll-out-of-view problem: the status
dashboard stays visible regardless of chat scroll position.

src/status-box.js

@@ -96,6 +96,21 @@ class StatusBox extends EventEmitter {
     return post.id;
   }
 
+  /**
+   * Delete a Mattermost post.
+   * @param {string} postId
+   * @returns {Promise<void>}
+   */
+  async deletePost(postId) {
+    try {
+      await this._apiCall('DELETE', `/api/v4/posts/${postId}`);
+      if (this.logger) this.logger.debug({ postId }, 'Deleted status post');
+    } catch (err) {
+      if (this.logger) this.logger.warn({ postId, err: err.message }, 'Failed to delete status post');
+      throw err;
+    }
+  }
+
   /**
    * Update a Mattermost post (throttled).
    * Leading edge fires immediately; subsequent calls within throttleMs are batched.

src/watcher-manager.js

@@ -274,9 +274,17 @@ async function startDaemon() {
     // Check if this session was previously completed (reactivation after idle)
     const completed = completedBoxes.get(sessionKey);
     if (completed) {
-      postId = completed.postId;
+      // Delete the old buried post and create a fresh one at the current thread position
+      // so users can see it without scrolling up
+      try {
+        await sharedStatusBox.deletePost(completed.postId);
+        logger.info({ sessionKey, oldPostId: completed.postId }, 'Deleted old buried status box on reactivation');
+      } catch (err) {
+        logger.warn({ sessionKey, oldPostId: completed.postId, err: err.message }, 'Failed to delete old status box (may already be deleted)');
+      }
       completedBoxes.delete(sessionKey);
-      logger.info({ sessionKey, postId }, 'Reactivating completed session — reusing existing post');
+      // postId stays null — will create a fresh one below
+      logger.info({ sessionKey }, 'Reactivating session — creating fresh status box');
     }
 
     // Check for existing post (restart recovery)