feat: add z command for smart directory jumping (frecency-based)

[ElianThorne]

• Implement z command with frecency scoring (count * 0.95^days_since_last_visit)
• Track directory visits automatically via cd hook
• Persist directory history using shared StateStore (avoids race condition)
• Add -l, -c, -r, -h, --add options for listing, filtering, and manual adds
• Add built-in mkdir and ls commands for reliable directory operations
• Improve cd with - (previous directory) support and path resolution
• Add ! prefix to run host OS commands (e.g., !pwd, !mkdir)
• Update internal _CURRENT_DIR state after cd and z to keep commands in sync
• Add comprehensive tests (29 tests passing)

Closes #11

[AkshajSinghal]

It's about time. The z command works, the code is clean, and the tests are decent.
One minor complaint: _cleanup_z_entries is a bit heavy, but it'll do for now.

Merging. Don't break it later.