fix: resolve read-only file system issues for Nix packaging

- Add config_paths.py for XDG-compliant path resolution
- Update all servers to use user-writable locations (logs, tokens, .env)
- Make .env file optional - server works with environment variables only
- Default paths: ~/.local/share for data, ~/.local/state for logs
- Update Nix wrapper with XDG defaults and helpful postInstall message
- Update README.md with environment variable configuration documentation

Fixes OSError when running in Nix store (read-only filesystem)

README.md

@@ -2,9 +2,48 @@
 
 This project provides a **FastMCP-powered** integration for Basecamp 3, allowing Cursor to interact with Basecamp directly through the MCP protocol.
 
-✅ **Migration Complete:** Successfully migrated to official Anthropic FastMCP framework with **100% feature parity** (all 46 tools)  
+✅ **Migration Complete:** Successfully migrated to official Anthropic FastMCP framework with **100% feature parity** (all 46 tools)
 🚀 **Ready for Production:** Full protocol compliance with MCP 2025-06-18
 
+## Configuration
+
+The Basecamp MCP server is configured entirely via environment variables. No `.env` file is required.
+
+### Required Environment Variables
+
+All required configuration is passed via environment variables:
+
+- `BASECAMP_CLIENT_ID` - OAuth client ID from 37signals
+- `BASECAMP_CLIENT_SECRET` - OAuth client secret from 37signals
+- `BASECAMP_ACCOUNT_ID` - Your Basecamp account ID
+- `BASECAMP_REDIRECT_URI` - OAuth callback URL (usually `http://localhost:8000/auth/callback`)
+- `USER_AGENT` - User agent string for API requests
+
+### Data Locations
+
+The server stores data in XDG-compliant locations:
+
+| File Type | Default Location | Environment Variable Override |
+|-----------|------------------|-------------------------------|
+| OAuth Tokens | `~/.local/share/basecamp-mcp/oauth_tokens.json` | `BASECAMP_TOKEN_FILE` |
+| Server Logs | `~/.local/state/basecamp-mcp/` | `BASECAMP_LOG_DIR` |
+
+### Optional .env File (Development Only)
+
+For local development, you can optionally create a `.env` file to avoid setting environment variables each time:
+
+```bash
+# Default location: ~/.config/basecamp-mcp/.env
+# Or override with: BASECAMP_ENV_FILE=/custom/path/.env
+BASECAMP_CLIENT_ID=your_client_id_here
+BASECAMP_CLIENT_SECRET=your_client_secret_here
+BASECAMP_ACCOUNT_ID=your_account_id_here
+BASECAMP_REDIRECT_URI=http://localhost:8000/auth/callback
+USER_AGENT="Your App Name (your@email.com)"
+```
+
+**Note:** The server works perfectly without a `.env` file - it's only a convenience for development.
+
 ## Quick Setup
 
 This server works with both **Cursor** and **Claude Desktop**. Choose your preferred client: