fix(model): prevent conversation model override snap-back after /model

[sarahwooders]

Summary

Fixes a conversation-scoped model regression where selecting a model with /model could revert back to the agent default after subsequent async state syncs.

What changed

• Add a sync-generation guard in src/cli/App.tsx so stale async model-sync runs can't overwrite newer state.
• Track recent per-conversation model override writes and skip local fallback-to-agent-default during a short freshness window.
• Mark override freshness on both /model selection and reasoning-tier flush paths.
• Add best-effort fallback in src/agent/modify.ts to conversations.retrieve after conversations.update if the update response omits model.
• Add a wiring regression test for the new guard behavior and update existing model wiring assertions.

Why

handleModelSelect correctly persists conversation overrides, but later sync passes (triggered by agent refresh / conversation sync) could run with stale timing and call applyAgentModelLocally, resetting UI state back to the agent model.

Testing

• bun test src/tests/agent/model-preset-refresh.wiring.test.ts
• bun test src/tests/cli/reasoning-cycle-wiring.test.ts
• bun test src/tests/cli/conversation-model-sync-guard.wiring.test.ts

Notes

There is an existing unrelated typecheck issue in App.tsx around compact params (compaction_settings.model), pre-existing in this branch base and not introduced by this PR.

[jnjpng]

#1238 might be a superset of this, gonna have discussion on that one if the race condition is legit on /model