feat(cli): add skill enable and disable management

[shuyhere]

Summary

• add /skill commands for listing, enabling, and disabling skills
• persist disabled skills in settings and apply them during runtime resource discovery
• add fullscreen support for managing skill state and test coverage for disabled-skill filtering

Why

This keeps skill management as an isolated feature PR instead of bundling it with fullscreen compaction or extension workflow changes.

Validation

• cargo test -p bb-cli slash::tests --quiet
• cargo test -p bb-cli extensions::tests --quiet