feat(install): add one-liner installation script for embedded binary

Add bash installation script that enables easy installation via:
curl -fsSL https://signal18.io/get-repman | bash

Features:
- Auto-detect OS (Linux/macOS) and architecture (amd64/arm64)
- Download latest release from GitHub with smart version detection
- Auto-fallback to ~/.local/bin if sudo not available
- Optional CLI client installation via REPMAN_INSTALL_CLI=true
- Comprehensive error handling and user feedback
- Post-installation verification

Files added:
- scripts/install.sh: Main installation script
- scripts/HOSTING.md: Web server configuration documentation
- scripts/README.md: Quick reference for developers

Environment variables:
- REPMAN_VERSION: Install specific version (default: latest)
- REPMAN_INSTALL_DIR: Custom installation directory
- REPMAN_INSTALL_CLI: Install CLI client (default: false)
- REPMAN_SKIP_VERIFY: Skip verification (default: false)

Supported platforms:
- Linux: amd64, arm64
- macOS: amd64 (Intel), arm64 (Apple Silicon)

Author: tanji