add @external decorator

packages/emitter/lib/decorators.tsp

···
 import "../dist/tsp-index.js";
 
+using TypeSpec.Reflection;
+
 /**
  * Specifies the maximum number of graphemes (user-perceived characters) allowed.
  * Used alongside maxLength for proper Unicode text handling.
···
  * ```
  */
 extern dec errors(target: unknown, ...errors: unknown[]);
+
+/**
+ * Marks a namespace as external, preventing it from emitting JSON output.
+ * This decorator can only be applied to namespaces.
+ * Useful for importing definitions from other lexicons without re-emitting them.
+ *
+ * @example
+ * ```typespec
+ * @external
+ * namespace com.atproto.repo.defs;
+ * ```
+ */
+extern dec external(target: Namespace);

packages/emitter/src/decorators.ts

···
 const inlineKey = Symbol("inline");
 const maxBytesKey = Symbol("maxBytes");
 const minBytesKey = Symbol("minBytes");
+const externalKey = Symbol("external");
 
 /**
  * @maxBytes decorator for maximum length of bytes type
···
 export function isReadOnly(program: Program, target: Type): boolean {
   return program.stateSet(readOnlyKey).has(target);
 }
+
+/**
+ * @external decorator for marking a namespace as external
+ * External namespaces are skipped during emission and don't produce JSON files
+ */
+export function $external(context: DecoratorContext, target: Type) {
+  if (target.kind !== "Namespace") {
+    context.program.reportDiagnostic({
+      code: "external-not-on-namespace",
+      severity: "error",
+      message: "@external decorator can only be applied to namespaces",
+      target: target,
+    });
+    return;
+  }
+  context.program.stateSet(externalKey).add(target);
+}
+
+export function isExternal(program: Program, target: Type): boolean {
+  return program.stateSet(externalKey).has(target);
+}

packages/emitter/src/emitter.ts

···
   isInline,
   getMaxBytes,
   getMinBytes,
+  isExternal,
 } from "./decorators.js";
 
 export interface EmitterOptions {
···
       for (const [_, childNs] of ns.namespaces) {
         this.processNamespace(childNs);
       }
+      return;
+    }
+
+    // Skip external namespaces - they don't emit JSON files
+    if (isExternal(this.program, ns)) {
       return;
     }
 

packages/emitter/src/tsp-index.ts

···
   $inline,
   $maxBytes,
   $minBytes,
+  $external,
 } from "./decorators.js";
 
 /** @internal */
···
     inline: $inline,
     maxBytes: $maxBytes,
     minBytes: $minBytes,
+    external: $external,
   },
 };