v3: collision-safe SHA-256 key encoding

[jeffwall-curlewlabs]

Summary

• Fix key collision bug: distinct keys like a/b and a:b previously sanitized to the same directory name a_b, silently returning wrong cache entries. Keys are now SHA-256 hashed into fixed-length (66-char) directory names, eliminating collisions and NAME_MAX issues with long keys.
• Fix portability: replaced GNU-only find -maxdepth/-mindepth with POSIX-compatible alternatives; added macOS to the CI test matrix.
• Extract shared helpers: encode_key, append_summary, and constants moved to lib/cache-common.sh, sourced by both scripts. CI test assertions also source this file so encoding stays in sync automatically.
• Preserve backward compatibility: existing v2/v1 cache entries are still found via legacy directory-name fallback. Prefix matching now scans entries and reads the stored raw key from a .local-cache-key metadata file, since SHA-256 hashes are not prefix-preserving.

This is a v3 major version because the on-disk cache layout changes. The README, SECURITY.md, and action.yml are updated accordingly, including an "Upgrading from v2" section.

Test plan

• shellcheck and sh -n pass on all three shell scripts
• Local smoke tests: collision safety (a/b vs a:b), dot keys (., ..), prefix matching, legacy v2 entry fallback, metadata file exclusion from restore targets
• CI passes on both ubuntu-latest and macos-latest (matrix added in this PR)

🤖 Generated with Claude Code