fix: support SOURCE_DATE_EPOCH for created metadata

[Huzaifa-Asif]

Summary

• honor SOURCE_DATE_EPOCH when generating the org.opencontainers.image.created OCI label and annotation
• keep {{date}} tag expressions based on the action’s current execution time
• validate SOURCE_DATE_EPOCH as a non-negative integer Unix timestamp
• document the environment variable and add coverage for valid, missing, invalid, and out-of-range values

Testing

• docker buildx bake test — 216 tests passed
• docker buildx bake pre-checkin — passed and regenerated the distribution files
• docker buildx bake validate — passed
• vitest run __tests__/meta.test.ts — 144 tests passed
• ESLint on the affected source and test files — passed
• tsc --noEmit — passed
• git diff --check — passed

Closes #403.

AI assistance

OpenAI Codex assisted with investigation, implementation, tests, and drafting.