feat(skill): wire bd mail delegate via bb shim

tests/skills/beadboard-driver/bb-mail-shim.test.ts

@@ -0,0 +1,27 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import path from 'node:path';
+import { execFile } from 'node:child_process';
+import { promisify } from 'node:util';
+
+const execFileAsync = promisify(execFile);
+const shimPath = path.resolve('skills/beadboard-driver/scripts/bb-mail-shim.mjs');
+
+test('bb-mail-shim fails with helpful error when bb is unavailable', async () => {
+  await assert.rejects(
+    async () => {
+      await execFileAsync(process.execPath, [shimPath, 'inbox'], {
+        env: {
+          ...process.env,
+          PATH: '',
+          BB_AGENT: 'silver-scribe',
+        },
+      });
+    },
+    (error: any) => {
+      const stderr = String(error.stderr || '');
+      assert.match(stderr, /bb command not found/i);
+      return true;
+    },
+  );
+});