fix(model-resolver): return variant from fallback chain, handle model name normalization

- Add variant to ModelResolutionResult return type
- Return variant from matched fallback entry
- Add normalizeModelName() for Claude model hyphen/period differences
- Add transformModelForProvider() for github-copilot model names
- Update delegate-task to use resolved variant (user config takes priority)
- Fix test expectations for new fallback behavior

src/agents/utils.test.ts

@@ -32,30 +32,30 @@ describe("createBuiltinAgents with model overrides", () => {
     expect(agents.Sisyphus.thinking).toBeUndefined()
   })
 
-  test("Sisyphus with systemDefaultModel GPT has reasoningEffort, no thinking", async () => {
+  test("Sisyphus uses first fallbackChain entry when no availableModels provided", async () => {
     // #given
     const systemDefaultModel = "openai/gpt-5.2"
 
     // #when
     const agents = await createBuiltinAgents([], {}, undefined, systemDefaultModel)
 
-    // #then
-    expect(agents.Sisyphus.model).toBe("openai/gpt-5.2")
-    expect(agents.Sisyphus.reasoningEffort).toBe("medium")
-    expect(agents.Sisyphus.thinking).toBeUndefined()
+    // #then - Sisyphus first fallbackChain entry is anthropic/claude-opus-4-5
+    expect(agents.Sisyphus.model).toBe("anthropic/claude-opus-4-5")
+    expect(agents.Sisyphus.thinking).toEqual({ type: "enabled", budgetTokens: 32000 })
+    expect(agents.Sisyphus.reasoningEffort).toBeUndefined()
   })
 
-  test("Oracle with default model has reasoningEffort", async () => {
-    // #given - no overrides, using systemDefaultModel for other agents
-    // Oracle uses its own default model (openai/gpt-5.2) from the factory singleton
+  test("Oracle uses first fallbackChain entry when no availableModels provided", async () => {
+    // #given - Oracle's first fallbackChain entry is openai/gpt-5.2
 
     // #when
     const agents = await createBuiltinAgents([], {}, undefined, TEST_DEFAULT_MODEL)
 
-    // #then - Oracle uses systemDefaultModel since model is now required
-    expect(agents.oracle.model).toBe("anthropic/claude-opus-4-5")
-    expect(agents.oracle.thinking).toEqual({ type: "enabled", budgetTokens: 32000 })
-    expect(agents.oracle.reasoningEffort).toBeUndefined()
+    // #then - Oracle first fallbackChain entry is openai/gpt-5.2
+    expect(agents.oracle.model).toBe("openai/gpt-5.2")
+    expect(agents.oracle.reasoningEffort).toBe("medium")
+    expect(agents.oracle.textVerbosity).toBe("high")
+    expect(agents.oracle.thinking).toBeUndefined()
   })
 
   test("Oracle with GPT model override has reasoningEffort, no thinking", async () => {