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.

tools/release_notes.py

@@ -29,6 +29,7 @@ VERSIONED_FILES = [
     "recover.sh",
     os.path.join("patch", "apply.sh"),
     os.path.join("patch", "create_task.py"),   # exposes `--version` to users
+    "update.sh",
 ]
 
 # `VERSION="x"` (shell) or `__version__ = "x"` (python).
@@ -139,7 +140,11 @@ def main(argv):
     version = argv[2]
 
     if cmd == "notes":
-        with open(CHANGELOG, encoding="utf-8") as fh:
+        # An explicit path lets update.sh show the notes from the CHANGELOG of the
+        # version it is about to install (`git show <tag>:CHANGELOG.md`), not the
+        # one already checked out.
+        path = argv[3] if len(argv) > 3 else CHANGELOG
+        with open(path, encoding="utf-8") as fh:
             print(extract_notes(fh.read(), version))
         return 0