fix: openclaw migration overwrites model config dict with string

[0xbyt4]

Summary

migrate_model_config() writes config["model"] = model_str which replaces the entire model dict with a bare string. This causes 'str' object has no attribute 'get' on every prompt after migration.

Root cause

# Before (bug): overwrites dict with string
hermes_config["model"] = model_str

# config.yaml becomes:
# model: "anthropic/claude-sonnet-4"   <-- string, loses provider/base_url

# Any code doing model_cfg.get("default") crashes

Fix

Preserve existing model dict, only update the default key:

existing_model = hermes_config.get("model")
if isinstance(existing_model, dict):
    existing_model["default"] = model_str
else:
    hermes_config["model"] = {"default": model_str}

Edge cases tested

• model is dict (normal) — updates default, preserves provider/base_url
• model is string (already broken) — converts to dict
• model key missing (fresh install) — creates dict
• model is None — creates dict

Affected users

Multiple reports in Discord — all from users who migrated from OpenClaw. Reinstalling without migration was the only workaround.