feat: support per-model thinkingDefault override in models config

The global `agents.defaults.thinkingDefault` forces a single thinking
level for all models.  Users running multiple models with different
reasoning capabilities (e.g. Claude with extended thinking, GPT-4o
without, Gemini Flash with lightweight reasoning) cannot optimise the
thinking level per model.

Add an optional `thinkingDefault` field to `AgentModelEntryConfig` so
each entry under `agents.defaults.models` can declare its own default.
Resolution priority: per-model → global → catalog auto-detect.

Example config:

    "models": {
      "anthropic/claude-sonnet-4-20250514": { "thinkingDefault": "high" },
      "openai/gpt-4o":                      { "thinkingDefault": "off" }
    }

Co-authored-by: Cursor <cursoragent@cursor.com>

src/config/types.agent-defaults.ts

@@ -18,6 +18,8 @@ export type AgentModelEntryConfig = {
   params?: Record<string, unknown>;
   /** Enable streaming for this model (default: true, false for Ollama to avoid SDK issue #1205). */
   streaming?: boolean;
+  /** Per-model default thinking level (overrides global thinkingDefault). */
+  thinkingDefault?: "off" | "minimal" | "low" | "medium" | "high" | "xhigh";
 };
 
 export type AgentModelListConfig = {