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/cli/tests/commands/describe.test.ts

@@ -89,6 +89,44 @@ describe('describe command', () => {
     expect(text).toContain('user-1');
   });
 
+  it('shows project Plugin Config with proxyModel', async () => {
+    const deps = makeDeps({
+      id: 'proj-1',
+      name: 'gated-project',
+      description: 'A gated project',
+      ownerId: 'user-1',
+      proxyModel: 'default',
+      proxyMode: 'direct',
+      createdAt: '2025-01-01',
+    });
+    const cmd = createDescribeCommand(deps);
+    await cmd.parseAsync(['node', 'test', 'project', 'proj-1']);
+
+    const text = deps.output.join('\n');
+    expect(text).toContain('Plugin Config:');
+    expect(text).toContain('Plugin:');
+    expect(text).toContain('default');
+    expect(text).not.toContain('Gated:');
+  });
+
+  it('shows project Plugin Config defaulting to "default" when proxyModel is empty', async () => {
+    const deps = makeDeps({
+      id: 'proj-1',
+      name: 'old-project',
+      description: '',
+      ownerId: 'user-1',
+      proxyModel: '',
+      gated: true,
+      createdAt: '2025-01-01',
+    });
+    const cmd = createDescribeCommand(deps);
+    await cmd.parseAsync(['node', 'test', 'project', 'proj-1']);
+
+    const text = deps.output.join('\n');
+    expect(text).toContain('Plugin Config:');
+    expect(text).toContain('default');
+  });
+
   it('shows secret detail with masked values', async () => {
     const deps = makeDeps({
       id: 'sec-1',