chore(repo): commit or ignore the untracked uv.lock #89

Context

The working tree contains an untracked uv.lock that is neither committed nor ignored (git check-ignore uv.lock returns nothing, git ls-files uv.lock is empty). It is not part of #84.

An untracked lockfile is ambiguous: contributors cannot tell whether dependency resolution is meant to be reproducible, and every git status reports noise.

Acceptance criteria

• Decide the dependency-locking policy.
  • Track it: commit uv.lock and (optionally) make CI install with uv sync --locked.
  • Or ignore it: add uv.lock to .gitignore.
• Note the decision in README.md / contributing docs if it affects local setup.