fix: skip registry sync when a specific package is requested

[yashas-hm]

Summary

• skills get <package> no longer clones or updates git registry repositories as a side-effect. Registry sync now only runs for an unfiltered skills get.
• Already-cloned local registries are still scanned during targeted installs, so they remain available if they happen to contain skills for the requested package.

Details

Previously, RegistrySync.sync() was called unconditionally, cloning all configured registries (e.g. the Flutter and Serverpod repos) into .dart_tool/skills/repos/ even when the user ran skills get jaspr. The registry skills were correctly filtered after cloning, but the cloning itself always happened, polluting the workspace with unrelated repos.

The fix guards the sync call behind if (packageNames == null), matching the intended semantics: a bare skills get (no args) fetches everything; skills get <package> only installs skills for that specific package without touching unrelated registries.

Test plan

• New test when installing a specific package then registries are not cloned verifies that a configured global registry is not cloned when skills get dep_pkg is run and dep_pkg has its own Dart skills.
• Existing registry tests (when git is unavailable, when installing from global registry) continue to pass.
• Full test suite passes (dart test).

Closes #5
Fixes #5