feat(setup): default to interactive Claude handoff on failure

Failures now launch an interactive Claude session instead of the
non-interactive assist (REASON/COMMAND parser). The user debugs
with full terminal access and types /exit to return to setup.

The original assist mode is available via --assist-mode flag or
NANOCLAW_SETUP_ASSIST_MODE=1 env var.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

setup/lib/windowed-runner.ts

@@ -18,7 +18,7 @@
 import * as p from '@clack/prompts';
 import k from 'kleur';
 
-import { offerClaudeAssist } from './claude-assist.js';
+import { offerClaudeOnFailure } from './claude-handoff.js';
 import { emit as phEmit } from './diagnostics.js';
 import type { StepResult, SpinnerLabels } from './runner.js';
 import { dumpTranscriptOnFailure, spawnStep, writeStepEntry } from './runner.js';
@@ -212,7 +212,7 @@ async function handleStall(
     // offerClaudeAssist runs its own spinner and may propose a fix command.
     // We don't attempt to restart the stalled build from here — if Claude
     // proposes a command the user accepts, they can retry setup afterwards.
-    await offerClaudeAssist({
+    await offerClaudeOnFailure({
       stepName,
       msg: `The ${stepName} step has produced no output for 60 seconds.`,
       hint: 'It may be hung on a slow network pull or a failing Dockerfile step.',