fix(tests): sync-integration cleanup wiped the shared reading_history table

sync_integration_test's cleanup ran DELETE FROM reading_history WHERE
true — harmless for years while the table sat empty, and a silent
stats-data destroyer the moment session capture started feeding it:
every suite run erased the shared dev accounts' captured reading
(the user's manual flip-through test data included).

The cleanup is now scoped: the sync tests' sessions belong to their
test-sync users, and deleting those users cascades their rows. Any
future table-wide cleanup DELETE is off the table (AGENTS.md test
rules updated).

AGENTS.md

@@ -53,7 +53,10 @@ were name-only across ALL libraries. Fixed; do not regress.
   broke on 2026-10-14 within hours of session capture shipping).
   Tests that need deterministic data use `setup.RegularToken` +
   `getRegularUserID` (the fresh per-test user), or assert ranges
-  instead of equals.
+  instead of equals. Related: **no table-wide DELETEs in test
+  cleanups** — sync_integration's `DELETE FROM reading_history WHERE
+  true` destroyed the live reader's stats data on every suite run
+  until scoped to the test-sync users (cascade via user delete).
 
 ## Timezones
 

cmd/server/tests/sync_integration_test.go

@@ -32,12 +32,16 @@ func setupSyncTestDB(t *testing.T) *database.Queries {
 
 	db := database.New(dbPool)
 	t.Cleanup(func() {
+		// Scoped wipes ONLY: reading_history now carries real captured
+		// sessions for the shared dev accounts, so a table-wide DELETE
+		// here silently destroyed a live reader's stats data on every
+		// suite run (2026-10-14). Sync-test sessions belong to the
+		// test-sync users; deleting those users cascades their rows.
 		_, _ = dbPool.Exec(ctx, "DELETE FROM sync_queue WHERE true")
-		_, _ = dbPool.Exec(ctx, "DELETE FROM reading_history WHERE true")
+		_, _ = dbPool.Exec(ctx, "DELETE FROM users WHERE email LIKE 'test-sync%'")
 		_, _ = dbPool.Exec(ctx, "DELETE FROM media_items WHERE title LIKE 'Test %'")
 		_, _ = dbPool.Exec(ctx, "DELETE FROM libraries WHERE name LIKE 'Test %'")
 		_, _ = dbPool.Exec(ctx, "DELETE FROM devices WHERE device_name LIKE 'Test %'")
-		_, _ = dbPool.Exec(ctx, "DELETE FROM users WHERE email LIKE 'test-sync%'")
 		dbPool.Close()
 	})