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.

CHANGELOG.md

@@ -1,5 +1,44 @@
 # Changelog
 
+## v0.4.0 — 2026-07-13
+
+### Added
+
+- **`update.sh`** — fetch a newer release and apply it, preserving your
+  nested-snapshot opt-in setting.
+
+  ```bash
+  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
+  ```
+
+  **Run it by hand. Never from cron or a systemd timer.** This patch injects
+  Python into middlewared and re-applies itself at every boot, so an unattended
+  pull would let any bad upstream commit reach your 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:
+
+  - **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 are ordered by **version**, not by date — date order silently downgrades
+    the box the first time a hotfix is tagged out of band (a v0.3.6 released 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.
+  - Shows the commits you don't have and the target's release notes (read from the
+    *target's* CHANGELOG, via `tools/release_notes.py` — not a second copy of the
+    extractor), then asks before doing anything.
+  - **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 quietly go stale
+  the way `create_task.py.__version__` did.
+
 ## v0.3.5 — 2026-07-13
 
 ### Changed