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/agent.js

@@ -10,6 +10,7 @@ import { queryClaudeSDK } from '../claude-sdk.js';
 import { spawnCursor } from '../cursor-cli.js';
 import { queryCodex } from '../openai-codex.js';
 import { Octokit } from '@octokit/rest';
+import { CLAUDE_MODELS, CURSOR_MODELS, CODEX_MODELS } from '../../shared/modelConstants.js';
 
 const router = express.Router();
 
@@ -634,9 +635,14 @@ class ResponseCollector {
  *                          - true: Returns text/event-stream with incremental updates
  *                          - false: Returns complete JSON response after completion
  *
- * @param {string} model - (Optional) Model identifier for Cursor provider.
- *                        Only applicable when provider='cursor'.
- *                        Examples: 'gpt-4', 'claude-3-opus', etc.
+ * @param {string} model - (Optional) Model identifier for providers.
+ *
+ *                        Claude models: 'sonnet' (default), 'opus', 'haiku', 'opusplan', 'sonnet[1m]'
+ *                        Cursor models: 'gpt-5' (default), 'gpt-5.2', 'gpt-5.2-high', 'sonnet-4.5', 'opus-4.5',
+ *                                       'gemini-3-pro', 'composer-1', 'auto', 'gpt-5.1', 'gpt-5.1-high',
+ *                                       'gpt-5.1-codex', 'gpt-5.1-codex-high', 'gpt-5.1-codex-max',
+ *                                       'gpt-5.1-codex-max-high', 'opus-4.1', 'grok', and thinking variants
+ *                        Codex models: 'gpt-5.2' (default), 'gpt-5.1-codex-max', 'o3', 'o4-mini'
  *
  * @param {boolean} cleanup - (Optional) Auto-cleanup project directory after completion.
  *                           Default: true
@@ -939,6 +945,7 @@ router.post('/', validateExternalApiKey, async (req, res) => {
         projectPath: finalProjectPath,
         cwd: finalProjectPath,
         sessionId: null, // New session
+        model: model,
         permissionMode: 'bypassPermissions' // Bypass all permissions for API calls
       }, writer);
 
@@ -959,7 +966,7 @@ router.post('/', validateExternalApiKey, async (req, res) => {
         projectPath: finalProjectPath,
         cwd: finalProjectPath,
         sessionId: null,
-        model: model || 'gpt-5.2',
+        model: model || CODEX_MODELS.DEFAULT,
         permissionMode: 'bypassPermissions'
       }, writer);
     }