docs(agents): Add dev server URLs and dev-ui proxy details (#112395)

## Summary
- Document the correct URLs for local dev servers
(`dev.getsentry.net:8000` and `sentry.dev.getsentry.net:7999`)
- Clarify that `pnpm run dev-ui` proxies API requests to production
sentry.io and requires no local backend
- Improve command comments to distinguish full devserver vs
frontend-only modes

## Test plan
- [ ] Verify URLs are correct by running `pnpm run dev-ui` and accessing
`https://sentry.dev.getsentry.net:7999/`

Author: sergical

AGENTS.md

@@ -130,13 +130,19 @@ make reset-db
 #### Development Setup
 
 ```bash
-# Start the development server
+# Start the full development server (requires devservices up)
 pnpm run dev
 
 # Start only the UI development server with hot reload
+# Proxies API requests to production sentry.io
 pnpm run dev-ui
 ```
 
+**Dev server URLs:**
+
+- Full devserver: http://dev.getsentry.net:8000
+- Frontend-only (`dev-ui`): https://sentry.dev.getsentry.net:7999/
+
 #### Typechecking
 
 Typechecking only works on the entire project. Individual files cannot be checked.