feat: add external rule source importer

[HexCodeYT]

Summary

Adds the first external rule-source compiler for PlainLink. The new plainlink-rules binary imports a conservative subset of ClearURLs JSON into PlainLink's existing .plainlink runtime format, so the cleaner can interoperate with mature rule ecosystems without changing clipboard/runtime code.

Details

• add plainlink-rules import-clearurls
• parse ClearURLs JSON without adding runtime dependencies
• import only safely representable providers: concrete domains plus simple exact/prefix query parameters
• skip providers with exceptions, wildcard TLDs, complete-provider rules, raw URL rules, redirections, and unsupported regex params
• add rules/sources.toml source metadata for ClearURLs, EasyList/uBO, and Firefox query stripping
• add importer fixture and integration tests
• document external source architecture, licensing posture, and future importer plan

Validation

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo run --bin plainlink-rules -- import-clearurls --input tests/fixtures/clearurls-sample.json --output /tmp/plainlink-clearurls-sample.plainlink --source-name 'ClearURLs Test Fixture' --source-url 'https://example.test/clearurls.json' --license test-only
• scripts/package-macos-app.sh

Notes

• This PR does not vendor generated third-party rules.
• Licensing/attribution metadata is documented, but generated external rules should stay out of the repo until redistribution terms are reviewed.