Automate releases from tags

Releases were manual and had drifted: v0.2.0 and v0.2.1 were tagged but never
released, so the releases page jumped v0.1.0 -> v0.3.0 and hid the fix for the
incident that took every app down.

Pushing a v* tag now runs the full suite and cuts a GitHub release whose body is
the matching CHANGELOG.md section -- one source of truth for release notes, so
there is no second place for them to be wrong.

The workflow refuses to publish when:
  - the tests, ruff, or bash -n fail (a tagged commit is what people install; it
    must be at least as good as main)
  - the tag does not match the VERSION= declared by every script
  - CHANGELOG.md has no section for the tag, or the section is empty

That version check is not theoretical: VERSION= had drifted to three different
values across install.sh / uninstall.sh / recover.sh / apply.sh and nothing
noticed until this release. tests/test_release_notes.py now asserts the scripts
agree with each other and with the newest CHANGELOG entry, so the drift cannot
come back.

workflow_dispatch takes an existing tag, so releases can be backfilled for tags
that were pushed before this existed.

106 tests, ruff and shellcheck clean.

CHANGELOG.md

@@ -83,6 +83,14 @@
   `.zfs/snapshot/<name>-<timestamp>/` path changes every run, which defeats
   restic's parent detection and forces a full re-scan each time.
 
+- **Automated releases.** Pushing a `v*` tag runs the full test suite and then
+  cuts a GitHub release whose body is the matching `CHANGELOG.md` section — so
+  release notes have exactly one source of truth. The workflow refuses to publish
+  if the tests fail, if the tag does not match the `VERSION=` declared by every
+  script, or if the CHANGELOG has no section for it. (`VERSION=` had silently
+  drifted to three different values across the scripts, and nothing noticed.)
+  `workflow_dispatch` can create a release for an already-existing tag.
+
 - **CI** (GitHub Actions): shellcheck + `bash -n` on every script, ruff, and
   pytest on Python 3.11/3.12/3.13. Includes tests that `compile()` the
   `*_BLOCK` strings — they are Python source appended to live middlewared