Add Deploy section to AGENTS.md (#2593)

* AGENTS.md: add Deploy section (main, 7 destinations, saas:enable)

Structured format for agent prelude discovery and cross-app
drift checking via verify-app-registry.

* Address review: document beta template env var requirement

The beta deploy config requires BETA_NUMBER; typical targets are beta1-beta4.

AGENTS.md

@@ -50,6 +50,14 @@ bin/jobs                     # Manage Solid Queue jobs
 bin/kamal deploy             # Deploy (requires 1Password CLI for secrets)
 ```
 
+## Deploy
+
+Default branch: `main`
+Pre-deploy: `bin/rails saas:enable`
+Deploy: `bin/kamal deploy -d <destination>`
+Destinations: production, staging, beta, beta1, beta2, beta3, beta4
+Note: `beta` is a template requiring `BETA_NUMBER` env var; typical targets are `beta1`-`beta4`.
+
 ## Architecture Overview
 
 ### Multi-Tenancy (URL-Based)