feat: add cache-matched-key output

[jackieli-tes]

Summary

• Add a new cache-matched-key output that reports the key of the cache entry that was found during restore
• When an exact match is found, cache-hit is true and cache-matched-key equals the input key
• When a restore-keys prefix match is found, cache-hit is false but cache-matched-key is set to the matching restore key
• On error paths, cache-matched-key is set to an empty string for consistency with cache-hit

Fixes #62

Changes

• src/utils.ts: Added setCacheMatchedKeyOutput helper function
• src/restore.ts: Call setCacheMatchedKeyOutput in all code paths (success, error, fallback)
• action.yml and restore/action.yml: Declare the new cache-matched-key output
• README.md: Added usage example showing restore-keys with cache-matched-key
• Rebuilt dist/ bundles

Test plan

• Verify exact key match sets cache-matched-key to the primary key
• Verify restore-key prefix match sets cache-matched-key to the matching restore key
• Verify failed restore sets cache-matched-key to empty string
• Verify fallback cache path sets cache-matched-key to the fallback matching key