Add dependency caching to CI

[mgoldsborough]

Summary

CI installs all dependencies from scratch on every run. Adding caching for uv and npm dependencies will make PR checks faster for contributors.

What to do

1. Edit .github/workflows/ci.yml
2. Add cache: true to the astral-sh/setup-uv step (uv supports built-in caching)
3. Add cache: 'npm' to the actions/setup-node step with cache-dependency-path: schemas/package-lock.json
4. Verify caching works on a second CI run (should show cache hits)

Acceptance criteria

• uv dependencies are cached between runs
• npm dependencies are cached between runs
• CI still passes cleanly

References

• setup-uv caching docs
• setup-node caching docs

Estimated time: ~30 minutes