Always rebuild the gateway in make build; migrate legacy providers by activation

internal/config/config.go

@@ -174,8 +174,13 @@ func (c *Config) normalize() {
 		case TargetClaude, TargetCodex:
 		default:
 			// Providers created before targets existed belong to the
-			// Claude page.
-			c.Providers[i].Target = TargetClaude
+			// Claude page, unless they were only ever activated for
+			// Codex.
+			if c.Providers[i].EnabledForCodex && !c.Providers[i].EnabledForClaude {
+				c.Providers[i].Target = TargetCodex
+			} else {
+				c.Providers[i].Target = TargetClaude
+			}
 		}
 	}
 	switch c.Gateway.PortMode {