feat(converters): centralize model field normalization across targets (#442)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/copilot-converter.test.ts

@@ -103,27 +103,9 @@ describe("convertClaudeToCopilot", () => {
     expect(parsed.body).toMatch(/## Capabilities\n- Threat modeling\n- OWASP/)
   })
 
-  test("agent model field is passed through", () => {
+  test("model field is dropped (Copilot model format differs from Claude model IDs)", () => {
     const bundle = convertClaudeToCopilot(fixturePlugin, defaultOptions)
     const parsed = parseFrontmatter(bundle.agents[0].content)
-    expect(parsed.data.model).toBe("claude-sonnet-4-20250514")
-  })
-
-  test("agent without model omits model field", () => {
-    const plugin: ClaudePlugin = {
-      ...fixturePlugin,
-      agents: [
-        {
-          name: "no-model",
-          description: "No model agent",
-          body: "Content.",
-          sourcePath: "/tmp/plugin/agents/no-model.md",
-        },
-      ],
-    }
-
-    const bundle = convertClaudeToCopilot(plugin, defaultOptions)
-    const parsed = parseFrontmatter(bundle.agents[0].content)
     expect(parsed.data.model).toBeUndefined()
   })