fix(metadata-protocol): user-facing copy for writable_package_required rejection

The rejection message is surfaced verbatim as a Studio toast, but its copy
was written for developers — it cited an internal ADR path as the remedy.
Rewrite the sentence as end-user remediation (switch to a writable package
in the package selector, or create one); the ADR pointer moves to a docs
property on the error. code/status/packageId are unchanged, so API
consumers and the ADR-0070 golden regression are unaffected.

Found by the Studio package-create UX dogfood (#2609, tracking #2615).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DHQc5BdGhTzPfazex3vWdt

Author: claude

.changeset/readonly-package-error-copy.md

@@ -0,0 +1,5 @@
+---
+"@objectstack/metadata-protocol": patch
+---
+
+Rewrite the `writable_package_required` rejection message as user-facing remediation ("switch to a writable package in the package selector, or create a new one") instead of developer-facing copy that cited an internal ADR path — the message is surfaced verbatim as a Studio toast. The ADR pointer moves to a `docs` property on the error; `code`, `status`, and `packageId` are unchanged.

packages/metadata-protocol/src/protocol.ts

@@ -3899,15 +3899,17 @@ export class ObjectStackProtocolImplementation implements ObjectStackProtocol {
                 request.packageId != null &&
                 !this.isWritablePackage(request.packageId)
             ) {
+                // Surfaced verbatim as a console toast — keep the sentence
+                // user-actionable; the ADR pointer lives in `docs` below.
                 const err = new Error(
-                    `[writable_package_required] Cannot author ${singularTypeForRepo}/${request.name} into `
-                    + `'${request.packageId}': it is a read-only code/installed package, not a writable base. `
-                    + `Create or select a writable base (package) first, then retry. `
-                    + `See docs/adr/0070-package-first-authoring.md.`,
+                    `[writable_package_required] Cannot save ${singularTypeForRepo}/${request.name}: `
+                    + `the package '${request.packageId}' is read-only (provided by code or an installed app). `
+                    + `Switch to a writable package in the package selector, or create a new one, and retry.`,
                 );
                 (err as any).code = 'writable_package_required';
                 (err as any).status = 422;
                 (err as any).packageId = request.packageId;
+                (err as any).docs = 'docs/adr/0070-package-first-authoring.md';
                 throw err;
             }
             const orgId = request.organizationId ?? null;