feat: Automatically create `.gitignore` in `.same` directory

[drlucaa]

To ensure build artifacts and local caches are not accidentally committed to version control, same should automatically initialize a .gitignore file within its internal metadata directory (.same/). This follows the "Reliable" and "Simple" brand pillars by providing sensible, secure defaults without requiring manual user intervention.

The implementation will reside in the internal/adapters/cas layer, ensuring that whenever a new Content Addressable Store is initialized, the corresponding ignore rules are also established.

Goals

• Prevent Data Leakage: Automatically exclude the local store/ and cache/ directories from Git.
• Centralized Configuration: Utilize the existing internal/core/domain/layout.go to define the .gitignore path and filename.
• Idempotent Execution: Ensure the file is only created if it does not already exist, allowing for potential future user overrides.

Acceptance Criteria

1. Domain Definition: internal/core/domain/layout.go contains a constant for .gitignore and a helper function DefaultGitIgnorePath().
2. Automatic Creation: When cas.NewStore() or cas.NewStoreWithPath() is called and creates the directory, it also writes a .gitignore file if one is missing.
3. File Content: The generated .gitignore contains exactly the following lines:

store/
cache/

4. Hermetic Verification:

• nix develop -c go test -race ./cli/internal/adapters/cas/... passes with new assertions.
• nix develop -c golangci-lint run ./cli/... passes without violations.

5. Unit Tests: Existing tests in store_test.go are updated to verify the physical presence and content of the .gitignore file after initialization.