fix: pass model parameter to Claude and Codex SDKs

Previously, the model parameter was accepted by the /api/agent endpoint
and extracted from requests, but was never passed through to the Claude
SDK or Codex SDK, causing all requests to use default models regardless
of user selection.

Changes:
- Add model parameter to queryClaudeSDK() options in routes/agent.js
- Add model to threadOptions in openai-codex.js
- Remove unused /cost slash command and PRICING constants
- Centralize all model definitions in shared/modelConstants.js
- Update API documentation to dynamically load models from constants

server/routes/cursor.js

@@ -6,6 +6,7 @@ import { spawn } from 'child_process';
 import sqlite3 from 'sqlite3';
 import { open } from 'sqlite';
 import crypto from 'crypto';
+import { CURSOR_MODELS } from '../../shared/modelConstants.js';
 
 const router = express.Router();
 
@@ -33,7 +34,7 @@ router.get('/config', async (req, res) => {
         config: {
           version: 1,
           model: {
-            modelId: "gpt-5",
+            modelId: CURSOR_MODELS.DEFAULT,
             displayName: "GPT-5"
           },
           permissions: {