Rework the Feishu frontend so each user turn renders as a single
collapsible task card that patches itself in place, replacing the
dq-based streaming path that produced many fragmented messages.
- One _TaskCard per turn; hook reacts to summary / exit_reason events
from the agent loop and patches the same card.
- Each step is a foldable panel: header shows the summary, expanding
reveals three sections (auto-hidden when empty):
* Thinking - from response.thinking (separate field, not content)
* Tool Calls - tool name + truncated JSON args
* Output - response.content, with protocol tags stripped so
the header summary is not duplicated inside
- Final reply rendered as a schema 2.0 markdown card for consistency.
- Code-review pass per code_review_principles.md:
* _TaskCard owns only stateful card lifecycle (start/step/done/fail)
* Pure formatting extracted to module-level _build_step_detail and
_fmt_tool_call (no more reaching into card._private from the hook)
* Hook is a ~10-line dispatcher
* Flattened a 4-level nested lambda into a named function
