Clear session links no carry-over made, at boot

Sessions from v0.2.8 and earlier were linked to the one before them by
every rollover and New session, so production threads offered "Show the
previous session" (and prompts read summaries) for sessions nothing was
carried from. A link is kept only when the session opens with a message
carried from that session.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/functions/backend/db/bot-sessions/clear-implicit-session-links-sql.test.ts

@@ -0,0 +1,32 @@
+import { Database } from "bun:sqlite";
+import { expect, test } from "bun:test";
+import clearImplicitSessionLinksSql from "./clear-implicit-session-links-sql";
+
+/**
+ * Only a carry-over keeps its link to the session before it. Run against
+ * real SQLite.
+ */
+test("clears links without a carried-over message", () => {
+    const db = new Database(":memory:");
+
+    db.run(`CREATE TABLE bot_sessions (id INTEGER PRIMARY KEY, previous_session_id INTEGER)`);
+    db.run(`CREATE TABLE bot_messages (id INTEGER PRIMARY KEY, session_id INTEGER, carried_from_session_id INTEGER)`);
+    db.run(`INSERT INTO bot_sessions VALUES (1, NULL), (2, 1), (3, 2), (4, 3)`);
+    db.run(`INSERT INTO bot_messages (id, session_id, carried_from_session_id) VALUES
+        (1, 2, 1),
+        (2, 3, NULL),
+        (3, 4, 1)`);
+
+    const { sql, values } = clearImplicitSessionLinksSql();
+
+    db.query(sql).run(...values);
+
+    const links = db.query(`SELECT id, previous_session_id FROM bot_sessions ORDER BY id`).all();
+
+    expect(links).toEqual([
+        { id: 1, previous_session_id: null },
+        { id: 2, previous_session_id: 1 },
+        { id: 3, previous_session_id: null },
+        { id: 4, previous_session_id: null },
+    ]);
+});