## What changed
- Add model metadata flags for enabling token budgeting and its history-notes extension.
- Apply those activation defaults once at thread startup when the user has not explicitly configured token budgeting, while respecting managed feature requirements.
- Continue resolving model-owned prompts for the active model so model switches can use their own token-budget guidance.
## Testing
- Update the token-budget integration test to verify that model metadata activates the feature without manual configuration.
GitOrigin-RevId: 73a32623eaeabb9243e240c43da7a5e7e93e9a19