bug: support post-GA v2 SemVer releases

[codeforester]

Summary

• define one repository-owned v2 SemVer policy for stable and approved prerelease identifiers
• apply it consistently to release orchestration, artifact build/verification, API manifests, and repository validation
• cover 2.0.1 and 2.1.0 refs, check, plan, notes, publish dry-run, and artifact paths while rejecting malformed or unsupported versions
• replace stale pre-GA help and v1 pinning guidance with the current verified v2 contract

Validation

• bats tests/release.bats tests/release-artifact.bats tests/api-manifest.bats
• ./tests/validate.sh (587 BATS tests plus all extended gates)

Fixes #349