feat(ext/setup): vault-presence probe helper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

extension/src/setup/probe.ts

@@ -0,0 +1,23 @@
+import type { GitHost } from '../service-worker/git-host';
+
+export interface VaultProbe {
+  exists: boolean;
+  lastCommit?: { sha: string; author: string; date: string };
+}
+
+/// Detect whether the configured remote already contains a relicario vault.
+/// Considered present if any of: .relicario/salt, .relicario/params.json,
+/// manifest.enc exists. Best-effort metadata fetch via lastCommit().
+export async function probeVault(host: GitHost): Promise<VaultProbe> {
+  const [relicarioFiles, rootFiles] = await Promise.all([
+    host.listDir('.relicario'),
+    host.listDir(''),
+  ]);
+  const exists =
+    relicarioFiles.includes('salt') ||
+    relicarioFiles.includes('params.json') ||
+    rootFiles.includes('manifest.enc');
+  if (!exists) return { exists: false };
+  const lastCommit = await host.lastCommit('manifest.enc');
+  return lastCommit ? { exists, lastCommit } : { exists };
+}