Session management UX gaps: ungrouped sessions can't be moved, cross-workspace sessions invisible, no UI delete

[Lijijun168]

When managing sessions across workspaces, the following pain points were encountered:

1. Ungrouped sessions cannot be moved into a workspace
   The only workaround is cwd auto-assignment or Forking the session. attachSession is rejected when cwd doesn't match the workspace path. No drag-and-drop or right-click "move to workspace" action exists.

2. Sessions belonging to another workspace are invisible
   There is no cross-workspace session list or search. A user may have sessions stuck in an abandoned workspace with no way to discover them through the UI — the only way to find them is by inspecting the filesystem directly.

3. No delete action for sessions in the UI
   Deleting a session requires manually deleting the session.jsonl.zstd file from the filesystem. There is no delete action in the session sidebar or any UI context menu.

4. No cross-workspace management at all
   Multiple workspaces on one machine operate in complete isolation. No global session list, no cross-workspace search.

Suggested improvements:

• Allow manually attaching a session to any workspace (override cwd validation or add a manual override flag)
• Add an "All Sessions" / global session list view
• Add a delete action to the session row context menu
• Add cross-workspace search in the sidebar