fix(config): preserve config data when validation fails

When readConfigFileSnapshot encounters validation errors, it now:
1. Returns the resolved config data instead of empty object
2. Uses passthrough() on main schema to preserve unknown fields

This prevents config loss when:
- User has custom/unknown fields
- Legacy config issues are detected but config is otherwise valid
- Zod schema does not recognize newer fields

Fixes config being overwritten with empty object on validation failure.

src/config/zod-schema.ts

@@ -1757,6 +1757,7 @@ export const ClawdbotSchema = z
       })
       .optional(),
   })
+  .passthrough()
   .superRefine((cfg, ctx) => {
     const agents = cfg.agents?.list ?? [];
     if (agents.length === 0) return;