feat(provider): add vercel to all gateway-supported fallback entries

Adds vercel to every fallback entry where the model exists on the
gateway (kimi-k2.5, glm-5, glm-4.6v, minimax-m2.7, grok-code-fast-1,
gpt-5-nano). Adds inferSubProvider mappings for minimax, moonshotai,
and zai. Updates librarian/explore special cases to prefer minimax
via gateway over claude-haiku.

src/shared/model-requirements.test.ts

@@ -40,7 +40,7 @@ describe("AGENT_MODEL_REQUIREMENTS", () => {
     expect(primary.variant).toBe("max")
 
     const second = sisyphus.fallbackChain[1]
-    expect(second.providers).toEqual(["opencode-go"])
+    expect(second.providers).toEqual(["opencode-go", "vercel"])
     expect(second.model).toBe("kimi-k2.5")
 
     const third = sisyphus.fallbackChain[2]
@@ -137,7 +137,7 @@ describe("AGENT_MODEL_REQUIREMENTS", () => {
     expect(primary.variant).toBe("medium")
 
     const secondary = multimodalLooker.fallbackChain[1]
-    expect(secondary.providers).toEqual(["opencode-go"])
+    expect(secondary.providers).toEqual(["opencode-go", "vercel"])
     expect(secondary.model).toBe("kimi-k2.5")
 
     const tertiary = multimodalLooker.fallbackChain[2]