fix(auth): keep resume lane warning non-blocking

- extract account resume lane guidance into a dedicated auth helper

- swallow diagnostic failures so account launches still reach execClaude

- add focused tests for the warning success and failure paths

tests/unit/auth/resume-lane-warning.test.ts

@@ -0,0 +1,53 @@
+import { describe, expect, it } from 'bun:test';
+import { maybeWarnAboutResumeLaneMismatch } from '../../../src/auth/resume-lane-warning';
+
+describe('resume lane warning', () => {
+  it('prints guidance when the plain ccs lane differs from the account lane', async () => {
+    const logs: string[] = [];
+
+    await maybeWarnAboutResumeLaneMismatch('work', '/tmp/account-lane', ['--resume'], {
+      log: (message) => logs.push(message),
+      resolvePlainLane: async () => ({
+        kind: 'native',
+        label: 'native Claude lane',
+        configDir: '/tmp/native-lane',
+        projectCount: 12,
+      }),
+    });
+
+    expect(logs[0]).toContain('Resume for account "work" will search that account lane');
+    expect(logs).toContain('[i]   Account lane: /tmp/account-lane');
+    expect(logs).toContain('[i]   Plain ccs lane: native Claude lane (/tmp/native-lane)');
+    expect(logs).toContain('[i]   Recover the original lane first: ccs -r');
+  });
+
+  it('does not log anything when resume is not requested', async () => {
+    const logs: string[] = [];
+
+    await maybeWarnAboutResumeLaneMismatch('work', '/tmp/account-lane', ['hello'], {
+      log: (message) => logs.push(message),
+      resolvePlainLane: async () => {
+        throw new Error('should not be called');
+      },
+    });
+
+    expect(logs).toHaveLength(0);
+  });
+
+  it('swallows diagnostic failures and keeps the warning path non-fatal', async () => {
+    const logs: string[] = [];
+
+    await expect(
+      maybeWarnAboutResumeLaneMismatch('work', '/tmp/account-lane', ['-r'], {
+        debug: true,
+        log: (message) => logs.push(message),
+        resolvePlainLane: async () => {
+          throw new Error('broken config');
+        },
+      })
+    ).resolves.toBeUndefined();
+
+    expect(logs[0]).toContain('Resume lane guidance skipped because diagnostics failed');
+    expect(logs[1]).toContain('Diagnostic error: broken config');
+  });
+});