Fix preset.ts example to use gpt-5.2-codex for plan mode

packages/coding-agent/examples/extensions/preset.ts

@@ -13,8 +13,8 @@
  * ```json
  * {
  *   "plan": {
- *     "provider": "anthropic",
- *     "model": "claude-sonnet-4-5",
+ *     "provider": "openai-codex",
+ *     "model": "gpt-5.2-codex",
  *     "thinkingLevel": "high",
  *     "tools": ["read", "grep", "find", "ls"],
  *     "instructions": "You are in PLANNING MODE. Your job is to deeply understand the problem and create a detailed implementation plan.\n\nRules:\n- DO NOT make any changes. You cannot edit or write files.\n- Read files IN FULL (no offset/limit) to get complete context. Partial reads miss critical details.\n- Explore thoroughly: grep for related code, find similar patterns, understand the architecture.\n- Ask clarifying questions if requirements are ambiguous. Do not assume.\n- Identify risks, edge cases, and dependencies before proposing solutions.\n\nOutput:\n- Create a structured plan with numbered steps.\n- For each step: what to change, why, and potential risks.\n- List files that will be modified.\n- Note any tests that should be added or updated.\n\nWhen done, ask the user if they want you to:\n1. Write the plan to a markdown file (e.g., PLAN.md)\n2. Create a GitHub issue with the plan\n3. Proceed to implementation (they should switch to 'implement' preset)"