VM setup: install uv and make it the Python convention

[modem7]

Summary

Adds uv to claude-code-vm-setup.sh, and makes it the Python convention in the generated CLAUDE.md.

Ubuntu 24.04's system Python refuses pip install (PEP 668, externally-managed-environment). The generated CLAUDE.md told Claude to force past that with --break-system-packages, which a live run hit straight away.

• install_uv (phase 2, after Rust) installs uv and uvx into ~/.local/bin from Astral's GitHub release for the machine's arch (x86_64 / aarch64). The download is checked against the release's published sha256 before installing, the same approach as the Go install.
  • Re-runs: skipped if already current, upgraded if older.
  • A uv installed somewhere else (snap, pipx, /usr/local/bin) is left alone.
  • If the latest-release lookup fails, an existing uv is kept with a warning.
• Generated CLAUDE.md now says to use per-project environments via uv (uv init + uv add, or uv venv + uv pip install, run with uv run), never to install into system Python, and to use uv tool install / uvx for Python CLI tools. The "Package managers" line lists uv instead of pip --break-system-packages.
• Health check: a new uv line.
• The webapp-testing skill's Playwright install keeps its --user --break-system-packages, now commented as the one deliberate exception: the skill's helper scripts run under plain python3.

uv also covers what pipx would (uv tool install, uvx), so pipx isn't added.

Test plan

• shellcheck clean
• install_uv in a fresh ubuntu:24.04 container:
  • fresh install → 0.12.19;
  • re-run → "already installed";
  • an older 0.12.0 in ~/.local/bin → upgraded to 0.12.19;
  • uv moved to /usr/local/bin → "managed elsewhere", untouched.
• As the user afterwards: uv venv + uv pip install six + import from the venv works