feat: add puppet forge remote type

Adds package: puppet for proxying the Puppet Forge API (forgeapi.puppet.com).

- remote/puppet.py: rewrites absolute forge URLs and relative /v3/files/
  paths in JSON responses to absolute proxy URLs; g10k uses
  url.ResolveReference so absolute file_uri values override the base
  entirely, meaning tarball downloads go straight to the proxy
- config.py: registers built-in mutable patterns for v3/modules/ and
  v3/releases (module metadata pages)
- artifact/proxy.py: dispatches to puppet.resolve_content for package:
  puppet remotes
- 9 new tests covering mutable detection, URL rewriting (relative and
  absolute), content-type, tarball pass-through, and pattern blocking

Client configuration (g10k):
  - config file: forge_base_url: https://artifacts.example.com/api/v1/remote/puppet-forge
  - Puppetfile: forge.baseUrl https://artifacts.example.com/api/v1/remote/puppet-forge

tests/conftest.py

@@ -112,6 +112,12 @@ TEST_REMOTES = {
             "immutable_patterns": [r"\.tgz$"],
             "cache": {"immutable_ttl": 0, "mutable_ttl": 1800},
         },
+        "puppet-test": {
+            "base_url": "https://forgeapi.puppet.com",
+            "package": "puppet",
+            "immutable_patterns": [r"^v3/files/.*\.tar\.gz$"],
+            "cache": {"immutable_ttl": 0, "mutable_ttl": 600},
+        },
     },
     "locals": {
         "local-test": {