feat(chat): image specialist legacy lane, Responses fixes, ACL + UI attachments

- Image expert: DashScope-style /chat/completions via image_legacy_client; early exit in
  direct_loop when skill_binding_role is image; shared placeholder helper; docs/IMAGE_SPECIALIST_LANE.md.
- Strict attachment ACL: link_attachment_acl on assistant chat_message rows (sqlite_store);
  chat attachment rate limit when user_id empty; admin chat tests updated.
- Admin chat UI: aggregate bubbles render assistant_text attachments (image_ref); WS expand path.
- turn_runner: persisted_chat_attachments_nonempty for final_msg selection.
- OpenAI Responses transport + agent_messages/agent_core_attempt adjustments; env docs and tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

runtime/chat/agent_messages.py

@@ -210,8 +210,10 @@ def build_llm_messages(
     :func:`~oclaw.runtime.direct_loop._guard_tool_results_for_llm_context`). Omit or leave empty
     to apply image-blob stripping for every tool row (safe default for callers without turn context).
 
-    Only the **last** user message may expand attachments into native multimodal ``input_image``;
-    older user attachments are replayed as text metadata only.
+    Only the **last** user message may expand attachments into native multimodal ``input_image``
+    blocks shaped like ``{"type":"input_image","image_base64","mime"}``; ``OpenAIChatModel`` and
+    ``OpenAIResponsesModel`` both accept this shape (see transport normalization code). Older user
+    attachments are replayed as text metadata only.
     """
     out: list[dict[str, Any]] = [{"role": "system", "content": (system_prompt or "").strip()}]
     dropped_unpaired_tool_rows = 0