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

src/auth/resume-lane-warning.ts

@@ -0,0 +1,64 @@
+import * as path from 'path';
+import { info, warn } from '../utils/ui';
+import {
+  parseResumeFlagIntent,
+  resolveRuntimePlainCcsResumeLane,
+  type ResumeLaneSummary,
+} from './resume-lane-diagnostics';
+
+interface ResumeLaneWarningDependencies {
+  resolvePlainLane?: () => Promise<ResumeLaneSummary>;
+  log?: (message: string) => void;
+  debug?: boolean;
+}
+
+export async function maybeWarnAboutResumeLaneMismatch(
+  profileName: string,
+  accountConfigDir: string,
+  args: string[],
+  deps: ResumeLaneWarningDependencies = {}
+): Promise<void> {
+  const resumeIntent = parseResumeFlagIntent(args);
+  if (!resumeIntent) {
+    return;
+  }
+
+  const log = deps.log ?? console.error;
+
+  try {
+    const plainLane = await (deps.resolvePlainLane ?? resolveRuntimePlainCcsResumeLane)();
+    if (path.resolve(plainLane.configDir) === path.resolve(accountConfigDir)) {
+      return;
+    }
+
+    log(
+      warn(
+        `Resume for account "${profileName}" will search that account lane, not the current plain ccs lane.`
+      )
+    );
+    log(info(`  Account lane: ${accountConfigDir}`));
+    log(info(`  Plain ccs lane: ${plainLane.label} (${plainLane.configDir})`));
+    if (resumeIntent.explicitSessionId) {
+      log(
+        info(
+          '  This explicit session ID may have been created in a different lane, so Claude may not find it here.'
+        )
+      );
+    }
+    log(info('  Recover the original lane first: ccs -r'));
+    log(info('  Back it up before changing setup: ccs auth backup default'));
+    log(
+      info(`  For future work, align plain ccs with this account: ccs auth default ${profileName}`)
+    );
+    log('');
+  } catch (error) {
+    log(
+      warn(
+        'Resume lane guidance skipped because diagnostics failed; continuing with the account lane.'
+      )
+    );
+    if (deps.debug ?? Boolean(process.env.CCS_DEBUG)) {
+      log(info(`  Diagnostic error: ${(error as Error).message}`));
+    }
+  }
+}