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/core/ports.ts

@@ -32,7 +32,9 @@ import type { UserSettings } from "./settings.ts";
 
 export interface ChatStore {
 	/** Create a session record (persisted immediately). */
-	createChat(title: string): ChatRecord;
+	/** Create a session record; kind "practice" marks a practice session
+	 *  (absent = teach, the default). */
+	createChat(title: string, kind?: "practice"): ChatRecord;
 	/** The full record, or null if unknown id / corrupt file. */
 	getChat(id: string): ChatRecord | null;
 	/** All sessions, newest activity first (the sidebar's list view). */