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

src/plugins/built-in/writing-studio/index.js

@@ -29,6 +29,7 @@ class WritingStudioPlugin extends PluginAPI {
 
     this._registerCommands(context);
     this._registerStatusBar(context);
+    this._registerExportFormats(context);
   }
 
   _registerCommands(context) {
@@ -120,6 +121,42 @@ class WritingStudioPlugin extends PluginAPI {
     });
   }
 
+  // Example usage of context.formats.registerExportFormat (Task 17): adds
+  // a "Writing Studio Summary" entry to the Export menu that writes a
+  // plain-text snapshot of today's sprint/goal progress instead of going
+  // through Pandoc. Doubles as documentation for how a plugin can offer
+  // its own export target.
+  _registerExportFormats(context) {
+    const { sprintEngine, goalTracker } = this;
+
+    context.formats.registerExportFormat('sprint-summary', {
+      label: 'Writing Studio Summary (.txt)',
+      extension: 'txt',
+      handler: async (markdownContent, outputPath) => {
+        const fs = require('fs');
+        const goal = context.settings.get('dailyGoal') || 1000;
+        const progress = goalTracker.getDailyProgress(goal);
+        const streak = goalTracker.getStreak(goal);
+        const wordCount = (markdownContent || '').split(/\s+/).filter(Boolean).length;
+
+        const lines = [
+          'Writing Studio Summary',
+          '=======================',
+          `Generated: ${new Date().toISOString()}`,
+          '',
+          `Document word count: ${wordCount}`,
+          `Daily goal: ${goal}`,
+          `Words written today: ${progress.written} (${progress.pct}%)`,
+          `Current streak: ${streak} day(s)`,
+          `Sprint active: ${sprintEngine.isActive() ? 'yes' : 'no'}`,
+          '',
+        ];
+
+        fs.writeFileSync(outputPath, lines.join('\n'), 'utf-8');
+      },
+    });
+  }
+
   deactivate() {
     if (this._sprintInterval) clearInterval(this._sprintInterval);
   }

src/plugins/built-in/writing-studio/manifest.json

@@ -32,7 +32,14 @@
         "shortcut": "Ctrl+Alt+G"
       }
     ],
-    "statusBar": { "indicators": ["sprint-timer", "word-goal"] }
+    "statusBar": { "indicators": ["sprint-timer", "word-goal"] },
+    "exportFormats": [
+      {
+        "id": "sprint-summary",
+        "label": "Writing Studio Summary (.txt)",
+        "extension": "txt"
+      }
+    ]
   },
   "settings": [
     { "key": "dailyGoal", "type": "number", "default": 1000, "label": "Daily word goal" },