docs: add INSTALL.md for cross-platform setup

INSTALL.md

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+# Installation Guide
+
+## Quick Start
+
+### Option 1: Install to system (recommended)
+
+```bash
+# Add Python to PATH if not already there
+export PATH="$HOME/.local/bin:$PATH"
+
+# Install tampy
+pip install tampy
+
+# Verify installation
+tampy --help
+
+# Use
+tampy run my_script.tampy
+tampy build my_script.tampy
+```
+
+### Option 2: Use virtual environment
+
+```bash
+# Create venv
+python -m venv .venv
+source .venv/bin/activate  # Windows: .venv\Scripts\activate
+
+# Install in development mode
+pip install -e ".[dev]"
+
+# Use
+tampy run my_script.tampy
+```
+
+## Windows (PowerShell)
+
+```powershell
+# Install
+pip install tampy
+
+# Add to PATH
+$env:APPDATA = [Environment]::GetFolderPath("ApplicationData")
+$env:PATH = "$env:APPDATA\Python\Scripts;" + $env:PATH
+
+# Verify
+tampy --help
+```
+
+## Development Setup
+
+```bash
+# Clone repository
+git clone https://github.com/yourusername/tampy.git
+cd tampy
+
+# Install with Makefile
+make install
+
+# Run tests
+make test
+
+# Build package
+make build
+
+# Clean
+make clean
+```
+
+## Troubleshooting
+
+### "tampy: command not found"
+
+Add venv bin to PATH:
+```bash
+# Linux/Mac
+export PATH="$HOME/.venv/bin:$PATH"
+
+# Windows (PowerShell)
+$env:PATH = "$env:APPDATA\Python\Scripts;" + $env:PATH
+```
+
+### Permission denied when installing
+
+```bash
+pip install --user tampy
+# or
+sudo pip install tampy  # Use with caution
+```
+
+## Cross-Platform Notes
+
+- **Linux**: Use `~/.local/bin` or venv bin
+- **macOS**: Use `~/Library/Python/3.x/bin` or venv bin
+- **Windows**: Use `%APPDATA%\Python\Scripts` or venv Scripts folder
+
+## Verify Installation
+
+```bash
+tampy --version
+tampy --help
+```