fix(update): count local changes one way for the preflight and the pull

The automatic update's preflight ignored mode-only changes and anything
whose status line contained plugins/ or plugin-repos/, then promised
"Automatic updates will not stash your changes". perform_core_update
used plain git status (modes count) and ignored only 'plugins/', then
ran 'git stash push -- :!plugins', which nothing ever pops. So an edit
to a bundled plugin under plugin-repos/, or the installer's chmods on
tracked scripts, passed the preflight and was stashed away for good.

- auto_update.local_changes() is the one predicate both use:
  core.fileMode=false, porcelain -z, and plugins/ and plugin-repos/
  excluded by leading folder rather than substring (a core file under
  web_interface/static/v3/js/plugins/ now counts).
- Update Code's explicit stash leaves out both plugin folders; the
  pull's --autostash carries their edits and mode changes across and
  reapplies them.
- The automatic updater calls perform_core_update(stash_local_changes=
  False), which refuses instead of stashing edits that appeared after
  the preflight; update_core reports that as 'blocked'.

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

docs/WEB_INTERFACE_GUIDE.md

@@ -102,7 +102,9 @@ Configure basic system settings:
   check are shown under the toggle, and anything other than success raises a
   banner on **Overview**.
   - *Checks first:* the code update is skipped, with the reason shown, if
-    tracked files were edited locally, the checkout has local commits, a
+    tracked files were edited locally (permission-only changes and edits under
+    `plugins/` or `plugin-repos/` don't count; the pull carries those across
+    and puts them back), the checkout has local commits, a
     rebase/merge is in progress, the branch has no upstream, less than 300 MB
     is free, or the newest version already failed once. A failed fetch is
     retried the next day.