fix: separate Reddit and Matrix runtime identities

tests/reddit-account-runtime.test.ts

@@ -10,6 +10,7 @@ import {
   RedditAccountRuntimeManager,
   type RedditAccountRuntimeResources,
 } from '../src/services/reddit-account-runtime.js';
+import { runtimeAuthenticatedAccount } from '../src/services/reddit-account-runtime-builder.js';
 
 function account(
   key: string,
@@ -219,6 +220,17 @@ describe('RedditAccountRuntimeManager', () => {
 });
 
 describe('account runtime isolation', () => {
+  it('uses the Reddit fullname for auth while retaining the Matrix runtime owner', () => {
+    expect(runtimeAuthenticatedAccount(
+      { matrixUserId: '@t2_1u57iqxxbv:reddit.com' },
+      { username: 'Upset_Marsupial1006', accountId: 't2_1u57iqxxbv' },
+    )).toEqual({
+      accountId: 't2_1u57iqxxbv',
+      redditUsername: 'Upset_Marsupial1006',
+      matrixUserId: '@t2_1u57iqxxbv:reddit.com',
+    });
+  });
+
   it('derives distinct deterministic browser profiles from account IDs', () => {
     const first = accountBrowserProfileDir('./data/browser', '@t2_first:reddit.com');
     const second = accountBrowserProfileDir('./data/browser', '@t2_second:reddit.com');