chore(settings): switch default model to claude-opus-4-6

Previously unset — Claude Code was defaulting to Opus 4.7 (1M) based on
plan capability + picker history. Pin the default to Opus 4.6 so future
sessions start on 4.6 without going through the /model picker.

1M context tier is plan-routed (Claude Max) so no explicit suffix needed
in the persisted ID — backup `.claude.json` files in this account show
the same bare form (`"model": "claude-opus-4-7"`) while runtime usage
logs as `claude-opus-4-7[1m]`.

Co-Authored-By: Claude <noreply@anthropic.com>

settings.json

@@ -276,6 +276,7 @@
     }
   },
   "effortLevel": "xhigh",
+  "model": "claude-opus-4-6",
   "remoteControlAtStartup": true,
   "skipAutoPermissionPrompt": true
 }