## 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
