docs: update README for plugin system, add proxyModel tests

- Rewrite README Content Pipeline section as Plugin System section
  documenting built-in plugins (default, gate, content-pipeline),
  plugin hooks, and the relationship between gating and proxyModel
- Update all README examples to use --proxy-model instead of --gated
- Add unit tests: proxyModel normalization in JSON/YAML output (4 tests),
  Plugin Config section in describe output (2 tests)
- Add smoke tests: yaml/json output shows resolved proxyModel without
  gated field, round-trip compatibility (4 tests)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/mcplocal/tests/smoke/proxymodel.test.ts

@@ -142,5 +142,40 @@ describe('ProxyModel smoke tests', () => {
       expect(output).toContain('Plugin Config');
       expect(output).toContain('Plugin:');
     });
+
+    it('mcpctl get projects -o yaml shows proxyModel and no gated field', async () => {
+      if (!available) return;
+
+      const output = await mcpctl('get projects -o yaml');
+      // proxyModel should be resolved (not empty)
+      expect(output).toContain('proxyModel:');
+      expect(output).not.toContain('gated:');
+    });
+
+    it('mcpctl get projects -o json shows proxyModel and no gated field', async () => {
+      if (!available) return;
+
+      const json = await mcpctl('get projects -o json');
+      const projects = JSON.parse(json) as Array<{ proxyModel?: string; gated?: boolean }>;
+      expect(projects.length).toBeGreaterThan(0);
+
+      for (const project of projects) {
+        expect(project.proxyModel).toBeDefined();
+        expect(project.proxyModel).not.toBe('');
+        expect(project).not.toHaveProperty('gated');
+      }
+    });
+
+    it('mcpctl get projects -o yaml is round-trip compatible with apply', async () => {
+      if (!available) return;
+
+      const yaml = await mcpctl('get projects -o yaml');
+      // Should contain kind and proxyModel (apply-compatible fields)
+      expect(yaml).toContain('kind: project');
+      expect(yaml).toContain('proxyModel:');
+      // Should not contain internal fields
+      expect(yaml).not.toContain('ownerId:');
+      expect(yaml).not.toContain('createdAt:');
+    });
   });
 });