fix: handle null head_commit and add error handling to release workflow

[ryanande]

Fixes issues in the release workflow:

Fixes:

• ✅ Handle null/empty head_commit.message (common when PRs are merged)
• ✅ Fix version bumping logic (proper string conversion)
• ✅ Check for existing tags before creating (prevents duplicate tag errors)
• ✅ Make all steps conditional to prevent errors

Improvements:

• Better error handling throughout
• More robust workflow that handles edge cases
• Prevents workflow failures on merge commits

This should resolve the errors from PR #28.