fix(tui): sync model preferences across clients

[opencode-agent]

Summary

• serialize model preference writes across TUI processes with the existing filesystem lock
• watch model.json so favorites, recents, and variants update in already-running TUIs
• apply preference changes as locked read-modify-write mutations so stale clients cannot overwrite unrelated changes

Validation

• bun test in packages/tui (735 passed, 5 skipped)
• bun typecheck in packages/tui
• bun test test/mini-host.test.ts in packages/cli
• bun typecheck in packages/cli

Requested by: @rekram1-node (Aiden via Slack)

[Enough1122]

AI code review — automated review for reference; please use your judgment.

1. packages/tui/src/model-preference.ts:139 — watcher.on("error") permanently closes the watcher with no retry, logging, or polling fallback — on filesystems where fs.watch fails (some network/overlay mounts) cross-client sync silently degrades to "never updates" and users get no signal why — at minimum log once; ideally fall back to periodic refresh() (e.g. every few seconds) when the watcher dies.

2. packages/tui/src/context/local.tsx:367 — mutations are applied optimistically (setPreferences(...)) and then persisted via repository.addRecent/setFavorite/saveVariant; a remote echo arriving between the optimistic set and the locked read-modify-write can momentarily revert the local UI (or drop the just-set value if the watcher fires with pre-write contents) — the revision guard prevents stale delivery but not this ordering window; consider tagging locally-initiated writes so their echo is distinguishable, or accepting the transient flicker explicitly.

3. packages/tui/src/model-preference.ts:126 — the watcher observes the entire directory of the preferences file and only filters by name; in a busy config/state directory this wakes the process on unrelated churn (and filename === undefined events always pass the filter) — cheap, but on high-event directories the 50 ms-debounced timer resets repeatedly and can starve refreshes; consider a max-deferral (e.g. fire after ≤500 ms regardless of further events).

4. Nit — packages/tui/src/model-preference.ts:104 — await import("@opencode-ai/util/flock") runs inside every update() call; hoist it to a lazily-cached module-level promise so the hot path stays allocation-free and failures aren't re-attempted per call.

5. Nit — packages/tui/test/model-preference.test.ts:70 — the subscription test relies on real fs events with a 2 s timeout; on CI runners without working inotify/FSEvents this becomes a guaranteed timeout failure rather than a skip — consider guarding on fs.watch availability or asserting via a direct refresh() invocation as well.

Overall: right fix — serializing read-modify-write through a file lock plus change broadcast solves the lost-update problem between concurrent TUI clients properly, and the moved helpers keep logic testable. Items 1–2 are worth a follow-up thought before merge. Thanks!