feat(fetchers): add PackageRegistryFetcher for PyPI, crates.io, npm (#65)

## What
Adds a `PackageRegistryFetcher` handling PyPI, crates.io, and npm
package URLs with structured metadata via their JSON APIs.

Closes #54

## Why
Agents evaluating dependencies need quick access to version, license,
and dependency info without parsing noisy registry HTML. Directly
supports the AGENTS.md requirement to avoid non-permissive licenses.

## How
- Single fetcher with sub-matching for three registries
- PyPI: `pypi.org/project/{name}` via `pypi.org/pypi/{name}/json`
- crates.io: `crates.io/crates/{name}` via
`crates.io/api/v1/crates/{name}`
- npm: `npmjs.com/package/{name}` via `registry.npmjs.org/{name}`
- Supports version-specific PyPI URLs and scoped npm packages
- Format field: `"package_registry"`

## Risk
- Low

### Checklist
- [x] Unit tests passed
- [x] Clippy clean
- [x] Formatting applied

Author: chaliy

crates/fetchkit/src/fetchers/mod.rs

@@ -8,6 +8,7 @@ mod docs_site;
 mod github_code;
 mod github_issue;
 mod github_repo;
+mod package_registry;
 mod stackoverflow;
 mod twitter;
 
@@ -16,6 +17,7 @@ pub use docs_site::DocsSiteFetcher;
 pub use github_code::GitHubCodeFetcher;
 pub use github_issue::GitHubIssueFetcher;
 pub use github_repo::GitHubRepoFetcher;
+pub use package_registry::PackageRegistryFetcher;
 pub use stackoverflow::StackOverflowFetcher;
 pub use twitter::TwitterFetcher;
 
@@ -135,6 +137,7 @@ impl FetcherRegistry {
         registry.register(Box::new(GitHubRepoFetcher::new()));
         registry.register(Box::new(TwitterFetcher::new()));
         registry.register(Box::new(StackOverflowFetcher::new()));
+        registry.register(Box::new(PackageRegistryFetcher::new()));
         // DocsSiteFetcher for docs sites and llms.txt
         registry.register(Box::new(DocsSiteFetcher::new()));
         // Default fetcher last (catches all remaining URLs)
@@ -293,14 +296,15 @@ mod tests {
     #[test]
     fn test_registry_with_defaults() {
         let registry = FetcherRegistry::with_defaults();
-        assert_eq!(registry.fetchers.len(), 7);
+        assert_eq!(registry.fetchers.len(), 8);
         assert_eq!(registry.fetchers[0].name(), "github_code");
         assert_eq!(registry.fetchers[1].name(), "github_issue");
         assert_eq!(registry.fetchers[2].name(), "github_repo");
         assert_eq!(registry.fetchers[3].name(), "twitter_tweet");
         assert_eq!(registry.fetchers[4].name(), "stackoverflow");
-        assert_eq!(registry.fetchers[5].name(), "docs_site");
-        assert_eq!(registry.fetchers[6].name(), "default");
+        assert_eq!(registry.fetchers[5].name(), "package_registry");
+        assert_eq!(registry.fetchers[6].name(), "docs_site");
+        assert_eq!(registry.fetchers[7].name(), "default");
     }
 
     #[test]