docs,skills: regenerate skill reference indexes; docs examples follow ADR-0112 casing

check:skill-refs and check:skill-examples gates — the skill reference
index derives from spec schemas (new error-code-ledger page), and the
prose-example compiler caught one remaining lowercase member literal.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MaSQn77TT5fUgHK9CesaDK

Author: claude

content/docs/plugins/development.mdx

@@ -273,7 +273,7 @@ import type { EnhancedApiError } from '@objectstack/spec/api';
 
 function handlePluginError(error: unknown): EnhancedApiError {
   return {
-    code: 'internal_error',
+    code: 'INTERNAL_ERROR',
     message: `Hello plugin error: ${String(error)}`,
     category: 'server',
     httpStatus: 500,

skills/objectstack-api/references/_index.md

@@ -20,7 +20,8 @@ from `node_modules` — there is no local copy in the skill bundle.
 
 ## Transitive dependencies
 
-- `node_modules/@objectstack/spec/src/api/contract.zod.ts` — The numeric HTTP status, when a producer chooses to mirror it into the body.
+- `node_modules/@objectstack/spec/src/api/contract.zod.ts` — Machine-readable semantic code (ADR-0112): a `StandardErrorCode` member or
+- `node_modules/@objectstack/spec/src/api/error-code-ledger.zod.ts` — Error-Code Ledger (ADR-0112 D3).
 - `node_modules/@objectstack/spec/src/api/realtime-shared.zod.ts` — Realtime Shared Protocol
 - `node_modules/@objectstack/spec/src/data/data-engine.zod.ts` — Data Engine Protocol
 - `node_modules/@objectstack/spec/src/data/filter.zod.ts` — Unified Query DSL Specification