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/setup-config.ts

@@ -123,6 +123,15 @@ export const CONFIG: Entry[] = [
     surface: 'flag',
     type: 'string',
   },
+  {
+    key: 'assistMode',
+    envVar: 'NANOCLAW_SETUP_ASSIST_MODE',
+    label: 'Assist mode',
+    help: 'Use non-interactive Claude assist on failure instead of interactive handoff.',
+    surface: 'flag',
+    type: 'boolean',
+    default: false,
+  },
 ];
 
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