Chatto API Compatibility

Read root AGENTS.md for the current release policy. Follow proto/AGENTS.md for shared API rules and the affected package's instructions for local requirements. Read ADR-045 for the decision history. Do not repeat these policies in the review.

1. Compare affected source with the target branch. For release work, also compare with the relevant released version.
2. Classify public changes as additive, behavioral, deprecated, or breaking.
3. State the effect on existing clients and stored data. Apply the current release policy; do not add compatibility work that it does not require.
4. Check behavior that a schema comparison cannot verify: authorization, errors, absence, validation, pagination, cursors, ordering, retries, and realtime negotiation or recovery.
5. Check generated clients, public documentation, and relevant tests. Run Buf breaking checks and tests for changed behavior. A passing schema check does not prove behavioral compatibility.

Keep public API changes separate from persisted chatto.core contracts. Review the local Operator API separately because it grants root-level access.

Report the classification, affected callers, required migration actions, checks run, and unresolved risks. For a breaking public change, check the approval, PR label, and documentation requirements in proto/AGENTS.md.