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.

src/store/chats.ts

@@ -31,7 +31,7 @@ function chatFile(id: string): string {
  *  phase, busy, pendingQuestion) is already announced by turn-started /
  *  turn-settled, which the sidebar refetches on. Silence here is law,
  *  not oversight. */
-function createChat(title: string): ChatRecord {
+function createChat(title: string, kind?: "practice"): ChatRecord {
 	const now = new Date().toISOString();
 	const record: ChatRecord = {
 		id: `chat_${Date.now().toString(36)}_${crypto.randomUUID().slice(0, 8)}`,
@@ -39,6 +39,9 @@ function createChat(title: string): ChatRecord {
 		createdAt: now,
 		updatedAt: now,
 		messages: [],
+		// Practice sessions carry the marker; teach stays ABSENT (absence
+		// reads as teach — pre-practice records never migrate).
+		...(kind === "practice" ? { kind: "practice" as const } : {}),
 	};
 	fs.mkdirSync(PATHS.chats, { recursive: true });
 	fs.writeFileSync(chatFile(record.id), JSON.stringify(record, null, "\t"), "utf-8");
@@ -73,6 +76,8 @@ function listChats(): ChatSummary[] {
 						// summaries; usage aggregates ride the full record, the picker
 						// never needs them).
 						...summary,
+						// Normalized kind: absent record field reads as teach.
+						kind: summary.kind ?? "teach",
 						messageCount: messages.length,
 						// Record-derived status for the session picker: phase from the
 						// transcript's explicit events (probe_done concluded / verdict