Add remote agent distribution: install scripts + binaries

- agent.ps1: Windows one-line install script (irm u-claw.org/agent.ps1 | iex)
- agent.sh: macOS/Linux install script (curl -fsSL u-claw.org/agent.sh | bash)
- downloads/: agent binaries for Windows, macOS, Linux
- vercel.json: add Content-Type headers for binary downloads

.gitignore

@@ -16,6 +16,7 @@ social-assets/
 .download-cache/
 *.dmg
 *.exe
+!website/downloads/agent.exe
 *.blockmap
 builder-debug.yml