feat(tui): expose Skills sync method setting

[SaladDay]

Summary

• expose the existing Skills sync method setting in the TUI Settings page
• match the GUI family design with the two upstream choices: Symlink and Copy Files
• treat the legacy Auto value as Symlink in the UI while saving an explicit choice
• add contextual help and focused interaction/rendering tests
• leave backend sync behavior unchanged

Validation

• cargo fmt --check
• cargo check --bin cc-switch
• cargo clippy --lib --no-default-features
• cargo test --lib -- --test-threads=1 (4163 passed, 2 ignored)
• focused TUI interaction and rendering tests
• two independent blind reviews completed with no findings