ci: make release tests post-GA-safe and exercise tags in CI

[codeforester]

Summary

• Make the release-test Git fixture own both absent and present local/remote tag lookups.
• Keep published v2.0.0 from affecting tests while retaining conflict and fail-closed coverage.
• Run the validation job from a full-history, tag-aware checkout.

Issue

Fixes #330

Validation

• bats tests/release.bats
• ./tests/validate.sh
• ./tests/lint-warnings.sh
• git diff --check

CI Impact

The validation matrix now checks out full history and tags, so release tests run against representative post-GA repository state.