feat(sources): add package definitions sync from GitHub (#40)

## Summary
- Add unified `DataLayerManager` that handles both sources and packages
with the same 3-layer override system (bundled → downloaded → user
custom)
- `santa sources update` now syncs both sources and packages from GitHub
- `santa sources clear` removes all downloaded data
- Deprecate `SourceLayerManager` in favor of `DataLayerManager`

## Test plan
- [x] `cargo test` passes
- [x] `cargo clippy` clean

Author: tylerbutler

codecov.yml

@@ -1,7 +1,7 @@
 codecov:
   require_ci_to_pass: yes
   notify:
-    after_n_builds: 2 # Wait for both unittests and datadriven uploads
+    after_n_builds: 1
     wait_for_ci: yes
 
 coverage: