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/openai-codex.js

@@ -213,7 +213,8 @@ export async function queryCodex(command, options = {}, ws) {
       workingDirectory,
       skipGitRepoCheck: true,
       sandboxMode,
-      approvalPolicy
+      approvalPolicy,
+      model
     };
 
     // Start or resume thread