Make the update_plan tool opt-in (#41744)

## What changed

- Default `tools.update_plan.enabled` to `false`; users can explicitly enable it to expose `update_plan`.
- Remove bundled `update_plan` guidance from model, collaboration-mode, multi-agent, compaction, prewarm, and goal-continuation prompts when the tool is disabled.
- Preserve custom base instructions, model catalog instructions, collaboration policies, and user goal text even when they mention planning or `update_plan`.

## Testing

- Cover default and explicitly enabled tool registration, prompt consistency across request paths, and preservation of custom instructions.

GitOrigin-RevId: a53964e6e72f98e2557dd3090fdb7caccb956527

codex-rs/core/config.schema.json

@@ -5546,7 +5546,7 @@
       "additionalProperties": false,
       "properties": {
         "enabled": {
-          "default": true,
+          "default": false,
           "type": "boolean"
         }
       },