feat: Implement GitHub Actions Caching for `.same` Directory

[drlucaa]

same relies on a robust caching strategy using a local Content Addressable Store (CAS) located at .same/store and tool resolution caches in .same/cache. Currently, CI/CD pipelines must rebuild the task graph and re-resolve Nix environments on every run, leading to slower build times.

This issue involves implementing a GitHub Action step in our CI workflow to persist the .same directory across runs. This will allow CI agents to share build metadata, input/output hashes, and resolved environment variable sets, significantly reducing compute time for incremental changes.

Goals

• Reduce CI Latency: Skip tasks that have already been executed with identical inputs by restoring the BuildInfo store.
• Persist Environments: Cache resolved Nix tool environments in .same/cache/environments to avoid expensive nix print-dev-env calls during Phase 1 hydration.
• Deterministic Keying: Ensure the cache key accurately reflects changes in tool definitions, dependency locks, and the Nix flake configuration.

Acceptance Criteria

• The .github/workflows/ci.yml file includes an actions/cache step that targets the .same directory.

• The cache key is generated using a hash of the following files to ensure determinism:

• **/same.yaml and **/same.work.yaml (Task definitions and tool aliases).

• cli/go.sum (Go dependency changes).

• flake.lock (Nix toolchain revisions).

• The cache is restored before the Generate Go source files and Run tests steps in the CI workflow.

• The cache is successfully updated/saved after a successful build execution.

• Verification that a "Cached Run" in CI contains "Skipped (Cached)" in the output for unchanged tasks.