Auto-install: prompt user to install uv + package if missing

When the sandbox package is not installed, the script now:
1. If uv is available: asks 'Install <pkg> with uv? [Y/n]'
2. If uv is missing: asks 'Install uv? [Y/n]', then installs the package
3. Falls back to pip if uv fails
4. Shows clear manual install instructions if all else fails

Add uv run --with for zero-config dependency management

- Both skills now use 'uv run --with <pkg>' in docs/examples
- Scripts show clear error + install instructions if package missing
- Discovery/instructions modes still work without any packages

Add hyperlight-codeact and monty-codeact skills