Add optional Windows unattended-run toolkit (hidden autostart + QR-recovery watchdog)

[jorgeschnura]

This PR adds an optional windows/ folder with a small, self-contained toolkit that makes the Go whatsapp-bridge run reliably unattended on Windows, plus a WINDOWS.md docs page linked from the existing "Windows Compatibility" section of the README.

It is purely additive: nothing that works today on macOS/Linux changes, and the Python MCP server and Go bridge are untouched. It only adds the Windows process-management layer around them.

Why Windows needs this

Running the bridge unattended on Windows hits three practical gaps:

1. The bridge must be a user process, not a service. whatsmeow links WhatsApp Web's multi-device session in the interactive user context. Run the bridge as a LocalSystem service and it starts, but its :8080 REST port never opens, so the MCP server has nothing to connect to. Running it as the logged-in user is the only configuration that reliably brings :8080 up.
2. A permanent visible console is undesirable. As a user process it normally leaves a terminal open forever (easy to close by accident). These scripts run it hidden.
3. The QR is only needed occasionally. WhatsApp sometimes unlinks a device and the bridge waits for a fresh QR. Showing a window on every restart is noise. The watchdog discriminates: it silently recovers ordinary crashes and only surfaces a visible QR window (plus a Windows notification) when bridge.log actually shows the session is unlinked.

What's added

• windows/start-whatsapp-hidden.vbs — launches the bridge hidden, fire-and-forget.
• windows/start-whatsapp-bridge.bat — auto-restart loop; UTF-8 console so a logged QR stays legible; writes bridge.log.
• windows/qr-bridge.bat — runs the bridge in a visible window so the QR is scannable.
• windows/whatsapp-bridge-watchdog.ps1 — recover-first watchdog (:8080 check, silent relaunch, QR window + notification only when truly unlinked).
• windows/install.ps1 — registers hidden autostart (at-logon scheduled task or Startup shortcut) + the watchdog scheduled task; parameterized with -WhatsAppMcpDir, with a matching -Uninstall.
• WINDOWS.md — step-by-step Windows setup, assuming the Go bridge is already built.
• A short pointer added under the README's existing "Windows Compatibility" section.

Notes

• No script hard-codes a path; they resolve the install dir from the WHATSAPP_MCP_DIR env var or their own location.
• Everything runs in the interactive user session by design; there is no headless/service mode, because :8080 only opens as a user process (that is the whole reason this exists).
• The watchdog matches the bridge's current QR log strings (Timeout waiting for QR, Scan this QR code); if that wording ever changes, one line updates.
• All PowerShell parses cleanly; MIT-licensed, consistent with the repo.

Happy to adjust naming, folder placement, or the README link to fit the project's conventions. A standalone version also lives at whatsapp-mcp-windows for users who find it before this lands.