Read the release id out of JSON, not out of a greedy regex

ci/publish-release.sh found the release id with

    sed -n -E 's/.*"id":[[:space:]]*([0-9]+).*/\1/p' | head -1

`.*` is greedy, the response is one line, and a Gitea release payload carries
an embedded "author" object and — once anything is attached — an "assets"
array, each with an id of its own. So it matches the LAST id, not the release's.

Measured against the four published releases here: their real ids are
416/417/419/421, and that pattern returns 1/2/3/4, which are the asset ids.

All four were nonetheless published correctly, and only by luck. The author id
on this instance is -1, the minus sign stops `[0-9]+` matching, and a create
response has no assets yet — so the last numeric id happened to be the
release's. A real author id, a Gitea that orders its fields differently, or a
re-run against a release that already has its zip, and it picks a number that
belongs to something else. Release 4 does not exist in this repository, so a
re-run of v1.4.3 today would 404 loudly rather than attach the zip to a
stranger's release, but that is the blast radius by accident, not by design.

Found by the session porting this into subscriptions, where the same line
picked the author id and the upload 404'd on its first real release.

json_field() reads one top-level field with python3, which release.yml already
requires. Verified against the live responses: 416 and 421 where the sed gives
1 and 4, and empty rather than a traceback on empty input, on "Not found." and
on an error object.

Two guards added with it, both verified against the live v1.4.3 release:

- the id's tag_name must equal the tag before anything is uploaded to it. The
  failure worth guarding is not a 404 — that is loud — but an id that exists
  and belongs to something else, which would attach this plugin's zip to an
  unrelated release with every check still green.
- a re-run skips the upload when the zip is already attached, instead of
  failing on the half that previously succeeded.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/REUSE.md

@@ -139,6 +139,15 @@ and a release with no zip attached both produce a page that looks perfect to
 anyone logged in, and an update that no shop is ever offered. A check that
 sends a token is the false positive that hides exactly that.
 
+**Do not parse a forge's JSON with `sed`.** A greedy `.*"id":([0-9]+)` matches
+the *last* id in the payload, and a release object carries an embedded
+`author` and, once anything is attached, an `assets` array — each with an id of
+its own. `ci/publish-release.sh` uses a four-line `json_field()` over `python3`
+instead. Measured here: the greedy pattern returns the asset ids 1/2/3/4 for
+releases whose real ids are 416/417/419/421. Four releases were published
+correctly anyway, purely because this instance's author id is **-1** and the
+minus sign stops `[0-9]+` from matching. That is not a safety margin.
+
 **A forge's own download URL is not the address your users can reach.**
 `browser_download_url` is built from the forge's configured root URL;
 `gitea.nolimit.dk` reports `http://57.129.89.117:25001/...` there. The updater