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/agents/model-selection.ts

@@ -425,10 +425,29 @@ export function resolveThinkingDefault(params: {
   model: string;
   catalog?: ModelCatalogEntry[];
 }): ThinkLevel {
+  // 1. Per-model thinkingDefault (highest priority)
+  // Normalize config keys via parseModelRef (consistent with buildModelAliasIndex,
+  // buildAllowedModelSet, etc.) so aliases like "anthropic/opus-4.6" resolve correctly.
+  const configModels = params.cfg.agents?.defaults?.models ?? {};
+  for (const [rawKey, entry] of Object.entries(configModels)) {
+    const parsed = parseModelRef(rawKey, params.provider);
+    if (
+      parsed &&
+      parsed.provider === params.provider &&
+      parsed.model === params.model &&
+      entry?.thinkingDefault
+    ) {
+      return entry.thinkingDefault as ThinkLevel;
+    }
+  }
+
+  // 2. Global thinkingDefault
   const configured = params.cfg.agents?.defaults?.thinkingDefault;
   if (configured) {
     return configured;
   }
+
+  // 3. Auto-detect from model catalog (reasoning-capable → "low")
   const candidate = params.catalog?.find(
     (entry) => entry.provider === params.provider && entry.id === params.model,
   );