When using `--session <UUID>`, the session lookup now:
1. Searches locally first (current project's session directory)
2. Falls back to global search across all projects
3. If found in different project, prompts user to fork the session
4. If not found anywhere, shows clear error instead of silently creating
a broken session with malformed path
Adds `SessionManager.forkFrom()` to create a forked session from another
project, preserving full conversation history with updated cwd.
48b4324155