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, 37 insertions, 0 deletions

diff --git a/src/features/chat/ui.test.ts b/src/features/chat/ui.test.ts
index f4006f7..aa43e04 100644
--- a/src/features/chat/ui.test.ts
+++ b/src/features/chat/ui.test.ts
@@ -1089,6 +1089,43 @@ describe("ModelSelector", () => {
     });
     expect(screen.getByText(/auto-described/i)).toBeInTheDocument();
   });
+
+  it("labels numbered OpenCode keys as OpenCode #N and sorts them numerically", () => {
+    const models = ["opencode-10/m1", "opencode-2/m2", "opencode-1/m3"];
+    const onSelect = vi.fn();
+    render(ModelSelector, {
+      props: { models, selected: "opencode-2/m2", onSelect },
+    });
+
+    const keySelect = screen.getByRole("combobox", { name: "Key selector" });
+    const options = within(keySelect).getAllByRole("option");
+    // Numeric order: #1 < #2 < #10, and values stay RAW (opencode-N).
+    expect(options.map((o) => (o as HTMLOptionElement).value)).toEqual([
+      "opencode-1",
+      "opencode-2",
+      "opencode-10",
+    ]);
+    expect(options[0]?.textContent).toContain("OpenCode #1");
+    expect(options[1]?.textContent).toContain("OpenCode #2");
+    expect(options[2]?.textContent).toContain("OpenCode #10");
+  });
+
+  it("shows a disappeared saved credential as unavailable without silently falling back", () => {
+    // Saved selection references opencode-5, but the catalog no longer has it.
+    const models = ["opencode-1/deepseek-v4-flash"];
+    const onSelect = vi.fn();
+    render(ModelSelector, {
+      props: { models, selected: "opencode-5/deepseek-v4-flash", onSelect },
+    });
+
+    // The unavailable key is surfaced as an explicit (disabled) option + notice.
+    const keySelect = screen.getByRole("combobox", { name: "Key selector" });
+    const unavailable = within(keySelect).getByText(/OpenCode #5 \(unavailable\)/i);
+    expect(unavailable).toBeInTheDocument();
+    expect(screen.getByText(/Selected model .* unavailable/)).toBeInTheDocument();
+    // No silent switch is attempted.
+    expect(onSelect).not.toHaveBeenCalled();
+  });
 });
 
 describe("ReasoningEffortSelector", () => {