Add configurable automatic presence to preserve phone notifications

[Info-Cado]

Summary

• add WUZAPI_AUTO_PRESENCE and -autopresence configuration with available and unavailable values
• use the configured presence for every session after connection and push-name synchronization
• document the Docker, Swarm, environment, and contact-presence behavior

Why

WuzAPI currently marks every linked session as available. WhatsApp can then treat the linked client as active and stop delivering push notifications to the primary phone.

Setting WUZAPI_AUTO_PRESENCE=unavailable keeps all WuzAPI sessions offline while still announcing their push names, allowing primary-phone notifications to continue. The default remains available for backward compatibility and for users who depend on contact presence updates.

Closes #350

Local validation

• go test ./...
• go vet ./...