docs: backfill 05bab55 journal entry; real cause in MIGRATION browser note

The migration session crashed before journaling. Follow-up diagnosis:
extensions vanished because programs.chromium.extensions wasn't carried
into the downstream flake — Chromium external-uninstall deletes the
extensions AND their Local Extension Settings. Rewrite the browser
migration note with the mechanism and the snapshot recovery path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/MIGRATION.md

@@ -267,9 +267,18 @@ default yet.
 - **Browser profiles:** Nomarchy does not manage Chromium/Firefox state.
   Bookmarks/extensions live under `~/.config/chromium` (or
   `~/.config/google-chrome` / ungoogled paths if that was your previous
-  browser). A package name change can look like "extensions vanished"
-  while bookmarks remain — point the new browser at the old profile dir
-  or re-install extensions; nothing was wiped by the activation.
+  browser). **If your old Home Manager config declared
+  `programs.chromium.extensions`, carry that block into `home.nix`
+  *before* first launch** — HM installs those via
+  `External Extensions/*.json`, and when the JSONs vanish Chromium
+  treats every extension as externally uninstalled and deletes it
+  **together with its stored data** (`Local Extension Settings` —
+  wallet vaults, password-manager pairings). Bookmarks survive, which
+  makes it look minor; it isn't. Recovery: re-declare the extension
+  ids, close the browser, and copy `Default/Local Extension
+  Settings/<id>` (plus any `Default/IndexedDB/chrome-extension_<id>_*`)
+  back from your Phase-0 `pre-nomarchy-home` snapshot before
+  relaunching.
 - **VPN:** NetworkManager connections survive under
   `/etc/NetworkManager` and your home. System › VPN lists NM
   `vpn`/`wireguard` profiles; import any that lived outside NM.