fix(security): ignore materialized Paperclip runtime directory

Adds .paperclip-runtime/ to the root .gitignore alongside the existing
.paperclip/ and .paperclip-local/ entries, and adds a Vitest regression
test that asserts git check-ignore covers adapter credential paths.

Without this entry, adapters that materialize runtime state into
<workspace>/.paperclip-runtime/<adapterKey>/... expose those files as
committable untracked entries inside the git working tree. A git add -A
while an auth.json is materialized would stage plaintext credentials.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.gitignore

@@ -45,6 +45,7 @@ diagnostics/
 .vscode/
 .claude/settings.local.json
 .paperclip-local/
+.paperclip-runtime/
 /.idea/
 /.agents/
 

packages/shared/src/gitignore-runtime.test.ts

@@ -0,0 +1,19 @@
+import { spawnSync } from "node:child_process";
+import { fileURLToPath } from "node:url";
+import { describe, expect, it } from "vitest";
+
+const repoRoot = fileURLToPath(new URL("../../..", import.meta.url));
+
+describe("repository gitignore", () => {
+  it("ignores materialized Paperclip runtime credentials", () => {
+    const sensitiveRuntimePath = ".paperclip-runtime/codex/home/auth.json";
+    const result = spawnSync("git", ["check-ignore", "-v", sensitiveRuntimePath], {
+      cwd: repoRoot,
+      encoding: "utf8",
+    });
+
+    expect(result.status).toBe(0);
+    expect(result.stdout).toContain(".paperclip-runtime/");
+    expect(result.stdout).toContain(sensitiveRuntimePath);
+  });
+});