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, 56 insertions, 0 deletions

diff --git a/src/features/chat/model-select.test.ts b/src/features/chat/model-select.test.ts
index 6d3081d..33c1a16 100644
--- a/src/features/chat/model-select.test.ts
+++ b/src/features/chat/model-select.test.ts
@@ -2,8 +2,11 @@ import { describe, expect, it } from "vitest";
 import {
   isVisionModel,
   joinModelName,
+  migrateLegacyModelName,
+  modelKeyLabel,
   modelKeys,
   modelsForKey,
+  openCodeNumber,
   splitModelName,
 } from "./model-select";
 
@@ -47,11 +50,64 @@ describe("modelKeys", () => {
     ).toEqual(["openai", "anthropic", "google"]);
   });
 
+  it("sorts numbered OpenCode keys numerically (#2 before #10)", () => {
+    expect(
+      modelKeys(["opencode-10/models", "opencode-2/models", "opencode-1/models", "openai/gpt-4"]),
+    ).toEqual(["opencode-1", "opencode-2", "opencode-10", "openai"]);
+  });
+
   it("is empty for no models", () => {
     expect(modelKeys([])).toEqual([]);
   });
 });
 
+describe("openCodeNumber", () => {
+  it("parses a numbered OpenCode key", () => {
+    expect(openCodeNumber("opencode-1")).toBe(1);
+    expect(openCodeNumber("opencode-10")).toBe(10);
+  });
+
+  it("returns null for non-openCode / malformed keys", () => {
+    expect(openCodeNumber("umans")).toBeNull();
+    expect(openCodeNumber("opencode")).toBeNull();
+    expect(openCodeNumber("opencode-0")).toBeNull();
+    expect(openCodeNumber("opencode-01")).toBeNull();
+  });
+});
+
+describe("modelKeyLabel", () => {
+  it("labels a numbered OpenCode key as OpenCode #N", () => {
+    expect(modelKeyLabel("opencode-2")).toBe("OpenCode #2");
+    expect(modelKeyLabel("opencode-10")).toBe("OpenCode #10");
+  });
+
+  it("leaves non-OpenCode keys unchanged", () => {
+    expect(modelKeyLabel("umans")).toBe("umans");
+  });
+});
+
+describe("migrateLegacyModelName", () => {
+  it("rewrites opencode/<model> to opencode-1/<model>", () => {
+    expect(migrateLegacyModelName("opencode/deepseek-v4-flash")).toBe(
+      "opencode-1/deepseek-v4-flash",
+    );
+    expect(migrateLegacyModelName("opencode/nested/model")).toBe("opencode-1/nested/model");
+  });
+
+  it("leaves already-numbered and other-provider names unchanged", () => {
+    expect(migrateLegacyModelName("opencode-1/deepseek-v4-flash")).toBe(
+      "opencode-1/deepseek-v4-flash",
+    );
+    expect(migrateLegacyModelName("umans/umans-glm-5.2")).toBe("umans/umans-glm-5.2");
+    expect(migrateLegacyModelName("opencode-2/x")).toBe("opencode-2/x");
+  });
+
+  it("is idempotent", () => {
+    const once = migrateLegacyModelName("opencode/deepseek-v4-flash");
+    expect(migrateLegacyModelName(once)).toBe(once);
+  });
+});
+
 describe("modelsForKey", () => {
   it("returns the model suffixes under a key, in order", () => {
     const models = ["openai/gpt-4", "anthropic/claude-3", "openai/gpt-4o"];