feat: reusable GitHub Actions for Nix cache setup and push

[principle-lgtm]

Summary

• Add .github/actions/setup — composite action: install Nix, configure substituter + trusted keys, set up auth for push
• Add .github/actions/push — composite action: sign and upload store paths or flake refs
• Update README with usage examples for consuming repos

Usage

- uses: stevedores-org/nix-cache/.github/actions/setup@develop
  with:
    push: true
    cache-auth-token: ${{ secrets.CACHE_AUTH_TOKEN }}
    signing-secret-key: ${{ secrets.NIX_SIGNING_SECRET_KEY }}
- run: nix build
- uses: stevedores-org/nix-cache/.github/actions/push@develop
  with:
    paths: .#default

Test plan

• CI passes on this PR
• Action YAML is valid (checked via actionlint or manual review)

Closes #6

🤖 Generated with Claude Code

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	nix-cache	2e14594	Commit Preview URL Branch Preview URL	Feb 25 2026, 11:27 PM