fix(core): remove unused finding categories

[Muawiya-contact]

Closes #70.

What changed

• Removed the dead DuplicateFile and EmptyDirectory category variants from diskern-core.
• Removed the matching CLI and desktop labels so the frontends only describe categories the engine can actually produce.
• Updated the stale exclude test expectation after the singular scan summary fix landed on main.
• Added a changelog note.

The duplicate report stays available through report.duplicate_sets, so users still see duplicate files in the dedicated duplicate section:

pub struct Report {
    pub duplicate_sets: Vec<dedup::DuplicateSet>,
}

Validation

cargo fmt --check
cargo test -p diskern-core -p diskern-cli --all-features
cargo clippy -p diskern-core -p diskern-cli --all-targets --all-features -- -D warnings
cd app && npm run build

[Muawiya-contact]

ok