fix: clear stale accepted commit marker after deleting local IndexedDB caches

When users delete local BME IndexedDB via UI actions (delete current/all
IDB), the chat metadata's st_bme_commit_marker was not cleared. This left
an accepted high-revision promise with no local DB backing, causing
persist-mismatch:indexeddb-behind-commit-marker and blocking graph load
indefinitely.

- index.js: add clearCurrentChatCommitMarker() helper and expose via runtime
- ui-actions-controller.js: call clearCurrentChatCommitMarker before
  syncGraphLoadFromLiveContext after IDB deletion
- p0-regressions.mjs: regression test asserting marker is cleared before
  reload after current-IDB deletion

ui/ui-actions-controller.js

@@ -1193,6 +1193,10 @@ export async function onDeleteCurrentIdbController(runtime) {
       });
     }
     runtime.clearCachedIndexedDbSnapshot?.(chatId);
+    runtime.clearCurrentChatCommitMarker?.({
+      reason: "manual-delete-current-idb",
+      immediate: true,
+    });
     runtime.syncGraphLoadFromLiveContext?.({
       source: "manual-delete-current-idb",
       force: true,
@@ -1245,6 +1249,10 @@ export async function onDeleteAllIdbController(runtime) {
     runtime.clearAllCachedIndexedDbSnapshots?.();
     const activeChatId = runtime.getCurrentChatId?.();
     if (activeChatId) {
+      runtime.clearCurrentChatCommitMarker?.({
+        reason: "manual-delete-all-idb",
+        immediate: true,
+      });
       runtime.syncGraphLoadFromLiveContext?.({
         source: "manual-delete-all-idb",
         force: true,