feat(plugins): add export-format registration hook to plugin API

Add context.formats.registerExportFormat(id, opts) to PluginContext,
backed by a new FormatRegistry (mirrors PluginRegistry's Map-based
shape). Plugins register namespaced (${pluginId}:${id}) export
formats with a label/extension/handler; the writing-studio built-in
plugin registers a trivial "sprint-summary" .txt export as a
worked example.

The plugin system lives entirely in the renderer process while the
Export menu is built in main.js, so wiring formats into the menu
required a small IPC round-trip: renderer sends format metadata to
main after plugin load (main rebuilds the menu via the already-
idempotent createMenu()), and a menu click sends the resolved save
path back to the renderer, which is the only process holding the
plugin's handler function.

Amit Haridas

tests/plugin-context.test.js

@@ -21,6 +21,7 @@ describe('PluginContext', () => {
       },
       ipc: { invoke: jest.fn(), on: jest.fn() },
       exportHooks: { preHooks: [], postHooks: [] },
+      formatRegistry: { register: jest.fn(), get: jest.fn(), getAll: jest.fn() },
     };
     context = new PluginContext(mockDeps);
   });
@@ -87,4 +88,18 @@ describe('PluginContext', () => {
     context.exports.registerPostHook(handler);
     expect(mockDeps.exportHooks.postHooks).toContain(handler);
   });
+
+  test('exposes formats.registerExportFormat with namespaced id', () => {
+    const handler = jest.fn();
+    const opts = { label: 'My Format', extension: 'txt', handler };
+    context.formats.registerExportFormat('my-format', opts);
+    expect(mockDeps.formatRegistry.register).toHaveBeenCalledWith('test-plugin:my-format', opts);
+  });
+
+  test('formats.registerExportFormat is a no-op when no formatRegistry is injected', () => {
+    const noRegistryContext = new PluginContext({ ...mockDeps, formatRegistry: undefined });
+    expect(() =>
+      noRegistryContext.formats.registerExportFormat('x', { handler: jest.fn() })
+    ).not.toThrow();
+  });
 });