feat(rules): classify npm download cache

[Muawiya-contact]

Closes #71.

What changed

• Added one rules-database entry for npm's download cache.
• Covers Unix/macOS npm cache files under ~/.npm/_cacache and Windows cache files under AppData\\Local\\npm-cache.
• Added tests for cache paths that should be safe and nearby npm state that should stay unknown.
• Added a changelog note.

Rule added:

{
  "id": "npm-cache",
  "patterns": ["**/.npm/_cacache/**", "**/appdata/local/npm-cache/**"],
  "category": "package_manager_cache",
  "verdict": "safe",
  "description": "npm's download cache. Re-fetched on demand; removing it only makes the next install download packages again."
}

Commits

• feat(rules): add npm cache rule
• test(rules): cover npm cache matching
• docs(changelog): note npm cache rule

Validation

cargo fmt --check
cargo test -p diskern-core --all-features npm
cargo test -p diskern-core -p diskern-cli --all-features
cargo clippy -p diskern-core -p diskern-cli --all-targets --all-features -- -D warnings

[Muawiya-contact]

ok