fix: clean up orphaned sessions with missing worktree paths on startup

[everpcpc]

Summary

When a worktree is deleted outside of the app (e.g., via git worktree remove in the terminal or manually removing the directory), the associated sessions remain in the database as orphans. This PR adds a startup cleanup step that detects and removes these orphaned sessions.

• SessionManager.cleanupOrphanedSessions() iterates all non-main-repo sessions and permanently deletes any whose worktree_path no longer exists on disk.
• Called once during app initialization in index.ts, right after initializeFromDatabase().

Tests

• No Test - Cleanup logic is straightforward filesystem existence check; pre-existing tests unaffected.

Type of change

• Bug Fix (non-breaking change which fixes an issue)