refactor(extension): one definition of what ships in the XPI

Milestone #271 step 1. Groundwork for deriving the extension version from git;
no behavior change yet -- nothing consumes `version` so far.

"Which files end up in the XPI" was stated in two places and about to become
three. Three hand-kept copies of one fact is what allowed #2397, where the
publish path could republish a stale XPI because its cache key had no link to
the content it stood for.

New extension/scripts/packaging.sh holds the single declaration and exposes:
  ignore       web-ext --ignore-files values
  pathspec     :(exclude)extension/... for git
  version      <MAJOR.MINOR from manifest>.<commit count over packaged files>
  major-minor  /  patch

Consumers now delegate instead of restating it:
- extension/package.json -- all four web-ext scripts
- .forgejo/workflows/ci.yml -- the extension-version guard's exclusions
- (step 4) the rev-list that derives the version

scripts/** joins the non-packaged set; the script must not ship to users.

Two shell hazards, both load-bearing:

The script runs `set -euf`. Its lists are iterated with deliberate word
splitting, and without -f the shell ALSO globs them -- invoking `pathspec`
from a directory where test/ exists (exactly how ci.yml calls it) would expand
`test/**` into the individual spec files and silently stop covering anything
added later. A caller's own `set -f` cannot prevent this: the script is a
separate sh process and does not inherit it.

Callers additionally need their own `set -f` for the substituted RESULT, which
is a different expansion. version.spec.js asserts every --ignore-files caller
sets it, that the pathspec comes through with `test/**` literal and no
.spec.js paths, and that neither consumer has reinstated a hardcoded list --
the easy future regression is "simplifying" by inlining one again.

Verified: all five subcommands plus the usage/exit-2 path. Derived version on
main (8300029) is 1.0.19, matching dev. Last published is 1.0.10, so the
eventual cutover moves strictly upward and needs no offset -- Firefox refuses
downgrades. (An earlier note recorded 18; that was measured against a stale
origin/main from before the PR #234 merge.)

Refs #2398

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

ci-requirements.md

@@ -49,7 +49,15 @@ per `docs/process.md`'s "add deps to the image when used by >1 project".
   classic script (`test/helpers/loadLib.js`) rather than adding `module.exports`
   shims to production code — the libs ship as `background.scripts`, not ES
   modules, so the specs exercise exactly the bytes packaged into the XPI.
-- Extension test files are excluded from the XPI via `--ignore-files` in
-  `extension/package.json`, and the same paths are excluded from `ci.yml`'s
-  `extension-version` guard. Those two lists must agree — `test/version.spec.js`
-  asserts the guard never ignores a file web-ext actually packages.
+- **`extension/scripts/packaging.sh` is the single definition of what ships
+  inside the XPI.** Three consumers read from it rather than keeping their own
+  copy: web-ext's `--ignore-files` (`extension/package.json`), the `:(exclude)`
+  pathspec in `ci.yml`'s `extension-version` guard, and the commit count that
+  derives the extension version. Three hand-kept copies of that one fact is
+  what allowed issue #2397.
+- Callers MUST `set -f` before substituting the script's output. Without it the
+  shell expands `test/**` against the working tree and silently narrows the
+  pattern to whatever files exist at that moment — a failure that looks like
+  nothing until dev files start appearing in the XPI. `test/version.spec.js`
+  asserts every `--ignore-files` consumer sets it, and that no consumer has
+  quietly reinstated a hardcoded list.