feat(models): numbered OpenCode credential selection, labels, migration

Adopt the numbered OPENCODE_API_KEY_N scheme in the model picker and
concurrency surfaces.

- model-select: numeric sort of opencode-<n> keys (#2 before #10); friendly
  OpenCode #N labels (raw opencode-N/<model> value still selected/persisted);
  migrateLegacyModelName rewrites opencode/<model> -> opencode-1/<model>.
- ModelSelector: renders friendly key labels; surfaces a disappeared saved
  credential as UNAVAILABLE (disabled option + notice) without silently
  switching to another model.
- app store: DEFAULT_MODEL -> opencode-1/...; catalogs no longer auto-fallback
  the active model when the selected credential is absent; boot restore
  migrates persisted legacy opencode/<model> tab models idempotently.
- concurrency: providerLabel renders opencode-<n> as OpenCode #<n> in the
  limit rows and Add dropdown (raw provider id preserved on the wire).
- Tests: parsing/sorting/labels/migration, unavailable-state selector behavior,
  no-silent-fallback, and concurrency provider label; fixtures updated to the
  numbered scheme.

Verification: bun install, bun run typecheck (0 errors), bun run test
(1179 passed), bun run check (clean, exit 0), bun run build (vite OK).

1 files changed, 7 insertions, 1 deletions

diff --git a/src/features/chat/index.ts b/src/features/chat/index.ts
index 2f98a0e..8cf776b 100644
--- a/src/features/chat/index.ts
+++ b/src/features/chat/index.ts
@@ -6,7 +6,13 @@ export type {
 } from "../../core/chunks";
 export { groupRenderedChunks, resolveImageUrl, viewProviderRetry } from "../../core/chunks";
 export type { TurnMetricsEntry } from "../../core/metrics";
-export { isVisionModel } from "./model-select";
+export {
+  isNumberedOpenCode,
+  isVisionModel,
+  migrateLegacyModelName,
+  modelKeyLabel,
+  openCodeNumber,
+} from "./model-select";
 export type { ChatTransport, HistorySync, HistoryWindow, MetricsSync } from "./ports";
 export type {
   EffortOption,