wip(13-09): checkpoint in-progress executor work before operator reboot

build-aiui.sh (mid-write) + aiui/.gitignore, committed verbatim and UNVERIFIED
— not a task completion. The 13-09 executor will be killed by the reboot; its
continuation should read this checkpoint, judge it against the plan's
must_haves, and reset --soft / build forward as appropriate (same recovery
pattern as the 6ba52b22/13b576da broken-pipe rescue at the start of this phase).
Already committed by 13-09 before this: 6ac0ebbf (CSP sandbox task).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

aiui/.gitignore

@@ -6,6 +6,12 @@ node_modules/
 dist/
 *.tsbuildinfo
 
+# scripts/build-aiui.sh's staleness-detection cache (13-09) — a local,
+# best-effort marker so the script can tell "source changed but the
+# emitted asset filenames didn't" from a plain rebuild with no changes.
+.build-aiui-last-src-hash
+.build-aiui-last-assets
+
 # Turborepo
 .turbo/