docs(scripts): add comprehensive installation scripts documentation

Document all 13+ installation scripts:
- Overview of multi-action support (install/update/uninstall/reconcile)
- Per-script detailed documentation
- Actions explained (install, update, uninstall, reconcile)
- Usage examples via Make targets
- Troubleshooting per script
- Shared utilities (lib/ directory)
- Best practices and patterns

Scripts covered:
- install_core.sh: fd, fzf, ripgrep, jq, yq, bat, delta, just
- install_python.sh: uv-based Python toolchain
- install_node.sh: nvm-based Node.js
- install_rust.sh: rustup-based Rust
- install_go.sh, install_aws.sh, install_kubectl.sh
- install_terraform.sh, install_ansible.sh, install_docker.sh
- install_brew.sh, install_uv.sh
- guide.sh: Interactive upgrade guide
- test_smoke.sh: Smoke testing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: AI Assistant