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/io.ts

@@ -296,7 +296,7 @@ export function createConfigIO(overrides: ConfigIoDeps = {}) {
           raw,
           parsed: parsedRes.parsed,
           valid: false,
-          config: {},
+          config: resolved as ClawdbotConfig,
           issues: validated.issues,
           legacyIssues,
         };