Fix: recreate sessions when resume fails with 'Session not found'

Worker sessions that were created but never received messages have no
events.jsonl, so the CLI server returns 'Session not found' on resume.
This caused multi-agent workers to silently vanish from the sidebar
after relaunch.

Now falls back to CreateSessionAsync when resume fails with 'Session
not found', preserving the session in _sessions so it remains visible
and functional.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: PureWeen

PolyPilot/Services/CopilotService.Persistence.cs

@@ -206,6 +206,24 @@ public async Task RestorePreviousSessionsAsync(CancellationToken cancellationTok
                         {
                             Debug($"Failed to restore '{entry.DisplayName}': {ex.GetType().Name}: {ex.Message}");
 
+                            // "Session not found" means the CLI server doesn't know this session
+                            // (e.g., worker sessions that were created but never received a message).
+                            // Fall back to creating a fresh session so multi-agent workers don't vanish.
+                            if (ex.Message.Contains("Session not found", StringComparison.OrdinalIgnoreCase))
+                            {
+                                try
+                                {
+                                    Debug($"Falling back to CreateSessionAsync for '{entry.DisplayName}'");
+                                    await CreateSessionAsync(entry.DisplayName, entry.Model, entry.WorkingDirectory, cancellationToken);
+                                    Debug($"Recreated session: {entry.DisplayName}");
+                                    continue;
+                                }
+                                catch (Exception createEx)
+                                {
+                                    Debug($"Fallback CreateSessionAsync also failed for '{entry.DisplayName}': {createEx.Message}");
+                                }
+                            }
+
                             // If the connection broke, recreate the client
                             if (ex is System.IO.IOException or System.Net.Sockets.SocketException
                                 or ObjectDisposedException