Runtime reliability: smoke tests + structured REST error taxonomy

[foo-bender]

Problem

Core chat/tool flows need stronger reliability guarantees and clearer operational diagnostics.

Proposal

• Add smoke/integration tests for key flows:
  • chat history persistence
  • command dispatch
  • tool-call lifecycle
• Standardize REST error payloads with stable machine-readable error codes.
• Improve release hygiene with changelog/release-note expectations.

Acceptance criteria

• Smoke tests cover critical user-facing flows.
• REST errors include consistent code/type fields.
• Changelog/release-note process documented.