feat(rules): classify npm download cache

CHANGELOG.md

@@ -6,6 +6,10 @@ All notable changes to Diskern are documented here. The format follows
 
 ## [Unreleased]
 
+### Added
+
+- npm download caches are now classified as safe package-manager cache files
+
 ### Fixed
 
 - The CLI scan summary now says "1 file" instead of "1 files" when a

crates/diskern-core/rules/base.json

@@ -57,6 +57,13 @@
       "verdict": "safe",
       "description": "pip download cache. Packages are re-downloaded on demand."
     },
+    {
+      "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."
+    },
     {
       "id": "unix-system-logs",
       "patterns": ["/var/log/**", "/private/var/log/**"],

crates/diskern-core/src/rules.rs

@@ -431,6 +431,38 @@ mod tests {
         }
     }
 
+    #[test]
+    fn npm_cache_is_safe_to_remove() {
+        let db = RulesDb::embedded();
+        for path in [
+            "/home/u/.npm/_cacache/content-v2/sha512/aa/package",
+            "/Users/u/.npm/_cacache/index-v5/ab/cd",
+            "C:\\Users\\u\\AppData\\Local\\npm-cache\\_cacache\\content-v2\\sha512\\aa\\package",
+        ] {
+            let (cat, verdict, rule) = db.classify(std::path::Path::new(path));
+            assert_eq!(
+                cat,
+                Category::PackageManagerCache,
+                "{path} matched {rule:?}"
+            );
+            assert_eq!(verdict, Verdict::Safe, "{path} matched {rule:?}");
+        }
+    }
+
+    #[test]
+    fn npm_state_outside_the_download_cache_is_not_marked_safe() {
+        let db = RulesDb::embedded();
+        for path in [
+            "/home/u/.npmrc",
+            "/home/u/.npm/_logs/2026-09-12-debug.log",
+            "C:\\Users\\u\\AppData\\Roaming\\npm\\npmrc",
+        ] {
+            let (cat, verdict, rule) = db.classify(std::path::Path::new(path));
+            assert_eq!(cat, Category::Unknown, "{path} matched {rule:?}");
+            assert_ne!(verdict, Verdict::Safe, "{path} matched {rule:?}");
+        }
+    }
+
     /// An installed application's own repair binary is not a reclaimable
     /// download. `unknown` is the right answer: report::build drops those,
     /// so it never reaches the user as an actionable row.