fix: block invalid config startup

Co-authored-by: Muhammed Mukhthar CM <mukhtharcm@gmail.com>

src/config/config.backup-rotation.test.ts

@@ -0,0 +1,36 @@
+import fs from "node:fs/promises";
+
+import { describe, expect, it } from "vitest";
+
+import { withTempHome } from "./test-helpers.js";
+import type { ClawdbotConfig } from "./types.js";
+
+describe("config backup rotation", () => {
+  it("keeps a 5-deep backup ring for config writes", async () => {
+    await withTempHome(async () => {
+      const { resolveConfigPath, writeConfigFile } = await import("./config.js");
+      const configPath = resolveConfigPath();
+      const buildConfig = (version: number): ClawdbotConfig =>
+        ({
+          identity: { name: `v${version}` },
+        }) as ClawdbotConfig;
+
+      for (let version = 0; version <= 6; version += 1) {
+        await writeConfigFile(buildConfig(version));
+      }
+
+      const readName = async (suffix = "") => {
+        const raw = await fs.readFile(`${configPath}${suffix}`, "utf-8");
+        return (JSON.parse(raw) as { identity?: { name?: string } }).identity?.name ?? null;
+      };
+
+      await expect(readName()).resolves.toBe("v6");
+      await expect(readName(".bak")).resolves.toBe("v5");
+      await expect(readName(".bak.1")).resolves.toBe("v4");
+      await expect(readName(".bak.2")).resolves.toBe("v3");
+      await expect(readName(".bak.3")).resolves.toBe("v2");
+      await expect(readName(".bak.4")).resolves.toBe("v1");
+      await expect(fs.stat(`${configPath}.bak.5`)).rejects.toThrow();
+    });
+  });
+});