feat: currentAuthor() carries git user.email — fail-open identity (F5 SLICE-2, #14)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/identity.ts

@@ -0,0 +1,25 @@
+/**
+ * Cross-rung identity (F5, spec §6.3 fork c): `email` is git's own join key —
+ * forges already map email → account. Resolved from the workspace git config,
+ * cached per root, FAIL-OPEN: absence just omits the optional field and the
+ * sidecar stays valid (spec §6.9). vscode-free, so it is unit-testable.
+ */
+import { execFileSync } from "node:child_process";
+
+const cache = new Map<string, string | undefined>();
+
+export function gitUserEmail(rootDir: string): string | undefined {
+  if (!cache.has(rootDir)) {
+    try {
+      const out = execFileSync("git", ["config", "user.email"], {
+        cwd: rootDir,
+        encoding: "utf8",
+        stdio: ["ignore", "pipe", "ignore"],
+      }).trim();
+      cache.set(rootDir, out || undefined);
+    } catch {
+      cache.set(rootDir, undefined);
+    }
+  }
+  return cache.get(rootDir);
+}