fix(openclaw): gate append on stored document text

[koriyoshi2041]

Fixes #2505.

Summary

• parse /version.features.store_document_text during the OpenClaw append-capability probe
• fall back to per-turn document ids when stored document text is explicitly disabled
• keep the existing version-only behavior when older /version payloads omit feature metadata

Tests

• npx vitest run src/index.test.ts
• npm run build
• npm test
• git diff --check

[koriyoshi2041]

Closing this in favor of #2511. I compared the two patches against #2505 and the current /version feature flag surface; #2511 keeps append gated on stored document text and covers the same OpenClaw path cleanly. Thanks!