fix: centralize model prefix handling to prevent provider errors

Moves prefix stripping from individual providers to AgentService/IdeationService
and adds validation to ensure providers receive bare model IDs. This prevents
bugs like the Codex CLI receiving "codex-gpt-5.1-codex-max" instead of the
expected "gpt-5.1-codex-max".

- Add validateBareModelId() helper with fail-fast validation
- Add originalModel field to ExecuteOptions for logging
- Update all providers to validate model has no prefix
- Centralize prefix stripping in service layer
- Remove redundant prefix stripping from individual providers

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/server/src/providers/cursor-provider.ts

@@ -28,7 +28,7 @@ import type {
   ModelDefinition,
   ContentBlock,
 } from './types.js';
-import { stripProviderPrefix } from '@automaker/types';
+import { validateBareModelId } from '@automaker/types';
 import { validateApiKey } from '../lib/auth-utils.js';
 import { getEffectivePermissions } from '../services/cursor-config-service.js';
 import {
@@ -317,8 +317,8 @@ export class CursorProvider extends CliProvider {
   }
 
   buildCliArgs(options: ExecuteOptions): string[] {
-    // Extract model (strip 'cursor-' prefix if present)
-    const model = stripProviderPrefix(options.model || 'auto');
+    // Model is already bare (no prefix) - validated by executeQuery
+    const model = options.model || 'auto';
 
     // Build CLI arguments for cursor-agent
     // NOTE: Prompt is NOT included here - it's passed via stdin to avoid
@@ -649,6 +649,10 @@ export class CursorProvider extends CliProvider {
   async *executeQuery(options: ExecuteOptions): AsyncGenerator<ProviderMessage> {
     this.ensureCliDetected();
 
+    // Validate that model doesn't have a provider prefix
+    // AgentService should strip prefixes before passing to providers
+    validateBareModelId(options.model, 'CursorProvider');
+
     if (!this.cliPath) {
       throw this.createError(
         CursorErrorCode.NOT_INSTALLED,