docs(persistence): pin why the local-only scrollback home stays outside full normalization

The two scrollback homes look symmetric (TERMINAL_SCROLLBACK_SESSION_HOMES), so the missing key
reads as an oversight. It is load-bearing: adding it would route the field through the fail-closed
strip and reintroduce the loss this branch fixes. The renderer prunes it with attribution before
the patch is sent, so the cap still holds without main as a second line.

src/main/persistence/loading-store/terminal-session-cleanup.ts

@@ -8,6 +8,11 @@ import {
   type TerminalScrollbackSnapshotStorage
 } from '../../terminal-scrollback-snapshots'
 
+// Why localOnlyScrollbackByTabId is deliberately NOT here despite TERMINAL_SCROLLBACK_SESSION_HOMES
+// pairing it with terminalLayoutsByTabId: full normalization reaches the fail-closed strip in
+// workspace-session-terminal-buffers.ts, which drops a leaf whose worktree main cannot attribute —
+// the renderer already capped that field with attribution in hand, and re-stripping it here is the
+// scrollback loss the local-only home exists to prevent. Add it only together with that strip.
 const WORKSPACE_SESSION_PATCH_FULL_NORMALIZATION_KEYS = new Set<keyof WorkspaceSessionState>([
   'tabsByWorktree',
   'terminalLayoutsByTabId'