Unstick plugins lazy.nvim won't update, and move to Thunderbird ESR

An interrupted plugin update can leave a plugin's files out of step with
its checkout, and lazy.nvim then refuses to update it, every time. Before
syncing, update now stashes such changes, so they can still be recovered
with git stash pop, and puts the plugin's submodules back.

Flathub retired org.mozilla.Thunderbird in favor of
org.mozilla.thunderbird_esr, so update tried to reinstall the old ID on
every run. Install the new one. flatpak update already moves existing
installs over; a migration carries a skip-packages entry for the old ID
over to the new one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/migrations.bats

@@ -0,0 +1,34 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+# Migrations that change files in the user's home
+
+SKIP="$BATS_TEST_TMPDIR/home/.config/coredesktop/skip-packages"
+THUNDERBIRD="$REPO/migrations/1794160046.sh"
+
+skip_list() {
+  mkdir -p "$(dirname "$SKIP")"
+  printf "$1" > "$SKIP"
+}
+
+@test "skipping the old Thunderbird ID carries over to the new one" {
+  skip_list 'foo\norg.mozilla.Thunderbird  # no mail'
+  run bash "$THUNDERBIRD"
+  [ "$status" -eq 0 ]
+  [ "$(cat "$SKIP")" = "$(printf 'foo\norg.mozilla.Thunderbird  # no mail\norg.mozilla.thunderbird_esr')" ]
+}
+
+@test "the Thunderbird migration leaves other skip lists alone" {
+  for list in 'org.mozilla.ThunderbirdX\n' 'org.mozilla.Thunderbird\norg.mozilla.thunderbird_esr\n'; do
+    skip_list "$list"
+    before="$(cat "$SKIP")"
+    run bash "$THUNDERBIRD"
+    [ "$status" -eq 0 ]
+    [ "$(cat "$SKIP")" = "$before" ]
+  done
+  rm -f "$SKIP"
+  run bash "$THUNDERBIRD"
+  [ "$status" -eq 0 ]
+  [ ! -e "$SKIP" ]
+}