fix: harden gstack-slug against shell injection via eval

Whitelist safe characters (a-zA-Z0-9._-) in SLUG and BRANCH output
to prevent shell metacharacter injection when used with eval.

Only affects self-hosted git servers with lax naming rules — GitHub
and GitLab enforce safe characters already. Defense-in-depth.

test/skill-validation.test.ts

@@ -793,6 +793,15 @@ describe('gstack-slug', () => {
     expect(lines[0]).toMatch(/^SLUG=.+/);
     expect(lines[1]).toMatch(/^BRANCH=.+/);
   });
+
+  test('output values contain only safe characters (no shell metacharacters)', () => {
+    const result = Bun.spawnSync([SLUG_BIN], { cwd: ROOT, stdout: 'pipe', stderr: 'pipe' });
+    const slug = result.stdout.toString().match(/SLUG=(.*)/)?.[1] ?? '';
+    const branch = result.stdout.toString().match(/BRANCH=(.*)/)?.[1] ?? '';
+    // Only alphanumeric, dot, dash, underscore are allowed (#133)
+    expect(slug).toMatch(/^[a-zA-Z0-9._-]+$/);
+    expect(branch).toMatch(/^[a-zA-Z0-9._-]+$/);
+  });
 });
 
 // --- Test Bootstrap validation ---