Fix: require uv in prereq check instead of silent pip fallback

[jacksandom]

Summary

• Make uv a hard requirement in check_deps() — fail immediately with an actionable error message instead of silently falling back to pip3/pip
• Remove dead pip/pip3 fallback codepath from setup_mcp() since uv is now enforced

Problem

When uv is not installed, the installer silently falls back to pip3/pip. This causes a cryptic setup.py not found or No module named pip error because:

1. The packages use pyproject.toml-only builds (no setup.py)
2. A venv created by a previous uv install doesn't include pip

The README already lists uv as a hard prerequisite, and the sub-project setup scripts (databricks-mcp-server/setup.sh, databricks-builder-app/scripts/setup.sh) already enforce it. The main install.sh was the only path that didn't.

Test plan

• Rename/hide uv from PATH, run bash install.sh — should fail immediately with clear install instructions
• Restore uv, run bash install.sh — should pass prereq check showing uv version

Closes #351