Fix bugs 2 and 3: agent panel expand and stale worktree references

Bug 2: Expand triangle now works when no agents are started - shows
"No agents started" message. AgentPanel moved to top of panels.

Bug 3: Run `git worktree prune` before `git worktree add` to clean
stale references from externally-deleted worktree directories.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/worktree.rs

@@ -95,6 +95,12 @@ fn create_worktree_sync(
             .map_err(|e| format!("Create worktree dir: {e}"))?;
     }
 
+    // Prune stale worktree references (e.g. directories deleted externally)
+    let _ = Command::new("git")
+        .args(["worktree", "prune"])
+        .current_dir(project_root)
+        .output();
+
     // Try to create branch. If it already exists that's fine.
     let _ = Command::new("git")
         .args(["branch", branch])
@@ -199,6 +205,57 @@ async fn run_teardown_commands(wt_path: &Path, config: &ProjectConfig) -> Result
     Ok(())
 }
 
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs;
+    use tempfile::TempDir;
+
+    /// Initialise a bare-minimum git repo so worktree operations work.
+    fn init_git_repo(dir: &Path) {
+        Command::new("git")
+            .args(["init"])
+            .current_dir(dir)
+            .output()
+            .expect("git init");
+        Command::new("git")
+            .args(["commit", "--allow-empty", "-m", "init"])
+            .current_dir(dir)
+            .output()
+            .expect("git commit");
+    }
+
+    #[test]
+    fn create_worktree_after_stale_reference() {
+        let tmp = TempDir::new().unwrap();
+        let project_root = tmp.path().join("my-project");
+        fs::create_dir_all(&project_root).unwrap();
+        init_git_repo(&project_root);
+
+        let wt_path = tmp.path().join("my-worktree");
+        let branch = "feature/test-stale";
+
+        // First creation should succeed
+        create_worktree_sync(&project_root, &wt_path, branch).unwrap();
+        assert!(wt_path.exists());
+
+        // Simulate external deletion (e.g., rm -rf by another agent)
+        fs::remove_dir_all(&wt_path).unwrap();
+        assert!(!wt_path.exists());
+
+        // Second creation should succeed despite stale git reference.
+        // Without `git worktree prune`, this fails with "already checked out"
+        // or "already exists".
+        let result = create_worktree_sync(&project_root, &wt_path, branch);
+        assert!(
+            result.is_ok(),
+            "Expected worktree creation to succeed after stale reference, got: {:?}",
+            result.err()
+        );
+        assert!(wt_path.exists());
+    }
+}
+
 async fn run_shell_command(cmd: &str, cwd: &Path) -> Result<(), String> {
     let cmd = cmd.to_string();
     let cwd = cwd.to_path_buf();