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-registry.test.js

@@ -1,6 +1,7 @@
 const { PluginRegistry } = require('../src/plugins/plugin-registry');
 const { PluginAPI } = require('../src/plugins/plugin-api');
 const { EventBus } = require('../src/plugins/event-bus');
+const { FormatRegistry } = require('../src/plugins/format-registry');
 
 class TestPlugin extends PluginAPI {
   init(context) {
@@ -153,4 +154,40 @@ describe('PluginRegistry', () => {
     registry.getPlugin('test').instance.ctx.exports.registerPreHook(handler);
     expect(registry.exportHooks.preHooks).toContain(handler);
   });
+
+  test('a plugin calling context.formats.registerExportFormat populates the injected FormatRegistry with a namespaced entry', () => {
+    const formatRegistry = new FormatRegistry();
+    const registryWithFormats = new PluginRegistry({ ...mockDeps, formatRegistry });
+
+    class ExportingPlugin extends PluginAPI {
+      init(context) {
+        context.formats.registerExportFormat('sprint-summary', {
+          label: 'Writing Studio Summary (.txt)',
+          extension: 'txt',
+          handler: async () => {},
+        });
+      }
+    }
+
+    registryWithFormats.register({
+      id: 'writing-studio',
+      name: 'Writing Studio',
+      version: '1.0.0',
+      description: 'desc',
+      manifest: {},
+      PluginClass: ExportingPlugin,
+      dir: '/tmp/test',
+    });
+
+    const entry = formatRegistry.get('writing-studio:sprint-summary');
+    expect(entry).toBeDefined();
+    expect(entry.label).toBe('Writing Studio Summary (.txt)');
+    expect(entry.extension).toBe('txt');
+    expect(typeof entry.handler).toBe('function');
+
+    const all = formatRegistry.getAll();
+    expect(all).toContainEqual(
+      expect.objectContaining({ id: 'writing-studio:sprint-summary', extension: 'txt' })
+    );
+  });
 });