v0.4.0: add update.sh

Fetch a newer release and apply it, preserving the nested-snapshot opt-in setting.

  bash update.sh              # to the newest release, with a confirmation
  bash update.sh --check      # show what would happen; change nothing
  bash update.sh --rollback   # undo the last update

Deliberately NOT automated. This patch injects Python into middlewared and
re-applies itself at every boot, so an unattended pull would let any bad upstream
commit reach a box with no human in the loop and take effect on the next reboot.
v0.0.4 shipped exactly such a bug and took every app on the box down. The manual
step is the safety gate.

Design decisions worth keeping:

- Defaults to the newest RELEASE TAG, not main. main can be mid-refactor; a tag is
  the tested artifact. --main exists but says so loudly.
- Tags ordered by version, not date. Date order silently downgrades the box the
  first time a hotfix is tagged out of band: a v0.3.6 cut after v0.4.0 would sort
  as "newest".
- Refuses to run over a dirty working tree rather than merging across hand-edited
  or scp'd files. (Verified: the guard fires.)
- Shows the commits and release notes you do not have, read from the TARGET's
  CHANGELOG via tools/release_notes.py -- not a second copy of the extractor.
- Records the previous revision BEFORE moving, so --rollback works even if
  install.sh dies halfway.
- Repairs .git ownership, which past `sudo git pull`s leave root-owned and which
  then breaks every later non-root git command.

update.sh is covered by the version-drift check, so it cannot go stale the way
create_task.py's __version__ did.

Tested end to end in a throwaway clone: detects v0.3.2 -> v0.3.5, lists missing
commits, handles already-up-to-date, and the dirty-tree guard fires.

132 tests, ruff and shellcheck clean.

install.sh

@@ -18,7 +18,7 @@
 
 set -euo pipefail
 
-VERSION="0.3.5"
+VERSION="0.4.0"
 
 # The directory containing install.sh is the permanent install location.
 PATCH_DIR="$(cd "$(dirname "$0")" && pwd)"
@@ -96,7 +96,7 @@ fi
 
 echo "Setting permissions ..."
 chmod +x "$PATCH_DIR/patch/apply.sh" "$PATCH_DIR/patch/create_task.py" \
-          "$PATCH_DIR/recover.sh" "$PATCH_DIR/uninstall.sh"
+          "$PATCH_DIR/recover.sh" "$PATCH_DIR/uninstall.sh" "$PATCH_DIR/update.sh"
 echo "Done."
 echo ""