Show tool and web search activity in the chat UI

Tool calls were invisible until the answer arrived: the only trace was the
sources list, and only for web searches. Surface them as they happen instead.

Add llm.stream_tool_calls(), an incremental version of resolve_tool_calls()
that yields a start and a completion event per tool call and ends with a
"resolved" event carrying the previous return value. resolve_tool_calls() is
kept as a thin wrapper that drops the progress events.

api/chat.py relays those events as a new SSE "tool" event, and wraps the
search-summary/knowledge-save step in one of the same shape, since it is a
noticeable pause of its own.

On the front end, ToolActivityList.vue renders one row per tool above the
assistant bubble (label, target, spinner while running, short result when
done). The store merges events by id and marks anything still running as
interrupted when the stream ends early. Display only - not persisted, so the
rows disappear when the session is reloaded, same as sources.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/architecture.md

@@ -146,6 +146,7 @@ LLM に渡すコンテキストは `memory_kb/memory.py` の `get_context_messag
 
 ```
 data: {"type":"session_id","value":"<uuid>"}   # 新規セッション時のみ
+data: {"type":"tool","value":{...}}             # ツール実行ごとに開始・完了の2回
 data: {"type":"notes_updated"}                  # AIが共有ノートを更新した時のみ
 data: {"type":"sources","value":[{...}]}        # Web検索した時のみ（出典の title/url）
 data: {"type":"chunk","value":"テキスト断片"}   # 0回以上
@@ -153,6 +154,13 @@ data: {"type":"done"}                           # 正常完了
 data: {"type":"error","value":"エラー内容"}     # 例外発生時
 ```
 
+`tool` イベントは `llm.stream_tool_calls()`（`resolve_tool_calls()` の逐次版）が yield する進行状況を
+そのまま中継したもので、`{id, name, label, detail, status}` で始まり、同じ `id` の
+`{id, status:"done", result}` で終わる。検索結果の要約・蓄積知識への保存も
+`id: "save_knowledge"` の同形式のイベントとして送る。フロント（`ToolActivityList.vue`）は
+`id` をキーにマージし、アシスタントバブルの上に実行中スピナーつきで一覧表示する。
+表示専用なので DB には保存せず、履歴を読み直すと消える。
+
 バッファリング防止のため `Cache-Control: no-cache` と `X-Accel-Buffering: no` を付与する。
 
 ---