Addresses all review items from @magyargergo and Copilot:
1. **Rename --no-git to --skip-git**: Commander.js treats --no-X flags
as negation of --X (stores as options.git = false, not options.noGit).
--skip-git maps correctly to options.skipGit.
2. **Fix false " Already up to date\ on non-git folders**: When
currentCommit is empty string, skip the cache check — we cannot
detect changes without git, so always rebuild.
3. **Replace isGitRepo() with hasGitDir()**: Use filesystem check
(statSync on .git) instead of shelling out to git CLI. Consistent,
faster, and works when git is not installed.
4. **Fix misleading warning**: Message now only fires when .git
directory is actually absent (not when git CLI fails).
5. **Add CLI integration tests**: Verify Commander maps --skip-git
correctly and that non-git folders are rejected without the flag.
