fix(tui): keep saved models across session transitions

[danielkov]

Summary

Keep a model saved as the default active for new sessions created by the running TUI. Upgrade agentkit-provider-openai to 0.10.8 so OpenAI sessions can replay continuation state across model changes without discarding encrypted reasoning or provider item IDs.

Motivation

The ACP backend snapshots its startup model, so creating another session after saving a new default could revert to the previous model until Kit restarted. Separately, exact model-binding validation rejected cross-model continuation locally even though the ChatGPT API accepts it for the model boundaries exercised.

Technical details

Saved model defaults

Track the saved model choice in the running TUI and apply it to newly created sessions before submitting their first prompt.

Cross-model continuation

Use the provider fix released from AgentKit #19, replacing this PR's continuation-discard workaround. Legacy continuation migration no longer compares the originating model with the active model; it preserves the originating model as bounded, non-empty provenance while retaining schema, session, item-kind, and authentication validation. The dependency update is limited to agentkit-provider-openai and its lockfile entry.

[kit-code-agent]

No issues found in the model-default session transitions or continuation-compatibility changes. The changes look good to merge. Validation: diff whitespace checks passed; focused OpenAI provider and TUI tests were attempted but could not run because compilation exhausted local disk space.

[kit-code-agent]

No issues found. The updated provider preserves cross-model continuation state while retaining session, authentication, schema, and item validation; legacy migration preserves bounded originating-model provenance. The dependency update is confined to agentkit-provider-openai 0.10.8. Validation passed: 16 OpenAI provider tests, 44 TUI tests, and diff whitespace checks.