feat: add npm remote type with metadata URL rewriting and caching

- Add `npm` package type to config with no built-in mutable defaults;
  users set explicit mutable_patterns (e.g. ^(?!.*\.tgz$).*) and
  immutable_patterns (e.g. \.tgz$) in remotes.yaml
- Rewrite dist.tarball URLs in metadata JSON on the fly so tarball
  downloads pass through the same proxy remote instead of hitting
  npmjs.org directly
- Single-remote design: npm_files_remote points back to itself since
  both metadata and tarballs are served from registry.npmjs.org
- Add .tgz to _get_content_type (application/gzip)
- Add example npm remote to remotes.yaml
- Add npm proxy section to README covering remotes.yaml config,
  client setup (npm/yarn/pnpm), rewriting behaviour, and
  mutable vs immutable path table
- Add tests for mutable pattern matching, URL rewriting, content-type,
  scoped packages, cache miss, and tarball immutability

tests/test_config.py

@@ -133,6 +133,44 @@ class TestGetMutablePatterns:
         assert r"repomd\.xml$" in patterns
         assert r"custom-meta\.xml$" in patterns
 
+    def test_npm_has_no_package_defaults(self, make_config):
+        cfg = make_config({"r": {"type": "remote", "package": "npm", "base_url": "https://x.com"}})
+        assert cfg.get_mutable_patterns("r") == []
+
+    def test_npm_explicit_mutable_pattern_matches_metadata(self, make_config):
+        import re
+
+        cfg = make_config(
+            {
+                "r": {
+                    "type": "remote",
+                    "package": "npm",
+                    "base_url": "https://x.com",
+                    "mutable_patterns": [r"^(?!.*\.tgz$).*"],
+                }
+            }
+        )
+        patterns = cfg.get_mutable_patterns("r")
+        assert any(re.search(p, "express") for p in patterns)
+        assert any(re.search(p, "@babel/core") for p in patterns)
+
+    def test_npm_explicit_mutable_pattern_excludes_tarballs(self, make_config):
+        import re
+
+        cfg = make_config(
+            {
+                "r": {
+                    "type": "remote",
+                    "package": "npm",
+                    "base_url": "https://x.com",
+                    "mutable_patterns": [r"^(?!.*\.tgz$).*"],
+                }
+            }
+        )
+        patterns = cfg.get_mutable_patterns("r")
+        assert not any(re.search(p, "express-4.18.2.tgz") for p in patterns)
+        assert not any(re.search(p, "express/-/express-4.18.2.tgz") for p in patterns)
+
 
 # ---------------------------------------------------------------------------
 # get_immutable_patterns