chore: remove requirements directory and add uv.lock

Deletes the entire requirements/ directory (all .txt lockfiles and
constraints.txt — the local constraints file has moved to constraints.txt
next to pyproject.toml). Adds the generated uv.lock which replaces all
the individual .txt lockfiles with a single universal lockfile.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: feanil