Add release workflow (manual dispatch)

[TGPSKI]

Summary

• Add .github/workflows/release.yml with workflow_dispatch trigger
• Supports patch, minor, major bump selection via dropdown
• Dry-run mode to preview version bump without side effects
• Verifies CI passed on HEAD before tagging
• Validates CHANGELOG.md has an entry for the target version
• Builds and runs tests as a final gate
• Creates annotated git tag and pushes to origin
• Creates GitHub release with changelog-extracted notes
• Triggers Go module proxy indexing and verifies pkg.go.dev availability
• All actions SHA-pinned following repo conventions

Design decisions

• Uses gh CLI (pre-installed on runners) instead of third-party release actions
• Requires CHANGELOG entry to exist before release — enforces changelog-driven releases
• Checks CI status on HEAD to prevent releasing broken code
• pkg.go.dev verification retries with backoff (5 attempts, 15s apart)
• Step summary provides clear version bump and release link output

Closes #11

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