feat(sessions): add Ctrl+D shortcut to delete sessions (#551)

[rhine9473]

Summary

Fixes #551.

This PR adds a Ctrl+D shortcut to delete sessions directly from the /sessions picker:

1. Shortcut & Binding: Pressing Ctrl+D on a selected session triggers the deletion workflow.
2. Confirmation Safeguard: Prompts with Delete session? Enter confirm · Esc cancel to prevent accidental deletion.
3. Active Session Protection: Blocks deleting the currently active session with an error notification.
4. Safe Deletion: Uses the system trash CLI when available, falling back to permanent unlink.
5. Real-Time UI Update: Removes the deleted session from the list in real-time, advances selection to adjacent session, and updates preview smoothly.
6. Unit Tests: Adds test coverage for deleteSessionFile under tests/extensions/sessions/sessions.test.ts.

Testing

• node --test --experimental-strip-types tests/extensions/sessions/sessions.test.ts (7 passed, 0 failed)
• Verified locally in interactive TUI on Windows: Ctrl+D triggers deletion confirmation, Enter deletes the file and refreshes list, Esc safely cancels.
• Biome check verified clean.