session kind: ChatRecord.kind teach|practice (absent = teach)

Practice sessions (owner, 2026-10-04): the marker rides the record and
the normalized summary (absent field reads as teach — pre-practice
records never migrate); POST /api/chats accepts {kind: 'practice'}.
The agent branch, tools, and UI land in the following slices.

tests/render-sidebar.test.tsx

@@ -21,6 +21,7 @@ function item(overrides: Partial<ChatListItem>): ChatListItem {
 	return {
 		id: "c1",
 		title: "Test session",
+		kind: "teach",
 		createdAt: "2026-01-01T00:00:00.000Z",
 		updatedAt: "2026-01-02T00:00:00.000Z",
 		messageCount: 4,