Validate session snapshots and wipe keys on logout (closes #10)

Client.fromJSON checks every snapshot field and each key's decoded length
and throws an error naming the bad field. toJSON reads the token through a
new ApiClient.getAuthToken and throws when there is none. logout zeroes the
key buffers in place; collectionsSince re-checks for logout after its
request so it never decrypts with zeroed keys. The CLI now reports a
corrupt session file separately from a missing one.

Model: opus-5-5

src/cli-session.ts

@@ -0,0 +1,26 @@
+// How the CLI reads its saved session file back into a `Client`.
+//
+// A missing file means "not logged in" and returns null. A file that exists but
+// cannot be read back into a client (bad JSON, a missing field, a key of the
+// wrong length) throws an error saying the session file is corrupt, so the CLI
+// can tell the user which of the two it is. Needs `init()` first.
+
+import { existsSync, readFileSync } from "node:fs";
+import type { ApiClientOptions } from "./api/client.js";
+import { Client } from "./client.js";
+
+export const loadSession = (
+    path: string,
+    apiOptions?: ApiClientOptions,
+): Client | null => {
+    if (!existsSync(path)) return null;
+    try {
+        return Client.fromJSON(
+            JSON.parse(readFileSync(path, "utf-8")),
+            apiOptions,
+        );
+    } catch (err) {
+        const reason = err instanceof Error ? err.message : String(err);
+        throw new Error(`Session file ${path} is corrupt: ${reason}`);
+    }
+};