Ship a lightweight reference application and validate independent adoption

Part of #83. Recommended work order: 12.

Dependencies: #87, #88, #89, #91, #92, #93, #94.

The primary onboarding journey currently demonstrates terminal chat. The product promise is an assistant integrated into an application through reusable contracts.

Scope:

- Build a lightweight reference frontend using the supported protocol/client.

- Demonstrate structured application context, an image, a read action, a change action, cancellation, and configurable evolving artifacts.

- Add a small non-browser host example using the same assistant contract.

- Update README, packaged docs, installation/release instructions, and examples around the Pydantic AI-first positioning.

- Run independent integration exercises and record the results.

Acceptance:

- [ ] A new developer can run the reference app from documented commands.

- [ ] Both hosts use public APIs with no runtime source edits.

- [ ] At least two unrelated assistant definitions run without inherited technical-operator personality.

- [x] Model and database smoke tests are separately runnable from deterministic CI. ([PR #117](https://github.com/eandualem/assistant-runtime/pull/117); database probe verified, live provider run still pending.)

- [ ] Restart/reconnect, rejected actions, artifact evolution, and cost visibility are demonstrated.

- [ ] Evidence records time to setup/first useful action, required assistance, core edits, task success, latency, and cost.

- [ ] Three unrelated integration exercises are documented, with at least one performed by a developer outside the original project.

- [ ] Public docs distinguish available features from future plans.

The reference frontend is an example client, not the only supported host. Begin its design early; completion depends on the public contracts. Suggested experimental targets are installation within 15 minutes and a first domain action within an hour; these are not current product promises.

Start by reviewing the existing terminal chat journey, supported protocol/client, and the dependency issues #87–#94 before designing the reference hosts. Use the documented commands and independent integration exercises to validate the acceptance checklist, including public-API use, restart/reconnect, actions, artifacts, cost visibility, and recorded adoption evidence.

Written by the indexing model from the issue text.