Default development environment to app.fizzy.localhost (#2822)

Using fizzy.localhost causes CORS errors when using minio for Active
Storage because the minio endpoint is at minio.fizzy.localhost — a
sibling subdomain, not a subdomain of the app host. Switching to
app.fizzy.localhost makes both hosts subdomains of fizzy.localhost,
resolving the CORS issue. See #2814 for the related minio CORS fix.

fizzy.localhost will continue to work if people want to use it, but all
docs and scripts have been updated to point to app.fizzy.localhost.

AGENTS.md

@@ -14,7 +14,7 @@ bin/setup              # Initial setup (installs gems, creates DB, loads schema)
 bin/dev                # Start development server (runs on port 3006)
 ```
 
-Development URL: http://fizzy.localhost:3006
+Development URL: http://app.fizzy.localhost:3006
 Login with: david@example.com (development fixtures), password will appear in the browser console
 
 ### Testing
@@ -156,7 +156,7 @@ Allow people to move between OSS and SAAS Fizzy instances:
 
 ### Chrome MCP (Local Dev)
 
-URL: `http://fizzy.localhost:3006`
+URL: `http://app.fizzy.localhost:3006`
 Login: david@example.com (passwordless magic link auth - check rails console for link)
 
 Use Chrome MCP tools to interact with the running dev app for UI testing and debugging.