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>

extension/package.json

@@ -3,11 +3,12 @@
   "version": "1.0.10",
   "private": true,
   "description": "Firefox extension for FabledCurator",
+  "comment_ignore_files": "The --ignore-files list comes from scripts/packaging.sh, the single source of truth shared with ci.yml's guard and the derived-version patch count. `set -f` is REQUIRED before the substitution: without it the shell globs `test/**` against the working tree and silently narrows the pattern to whatever files happen to exist.",
   "scripts": {
-    "lint":  "web-ext lint --source-dir=. --no-config-discovery --ignore-files package.json package-lock.json web-ext-artifacts node_modules README.md .gitignore vitest.config.js \"test/**\"",
-    "start": "web-ext run --source-dir=. --no-config-discovery --ignore-files package.json package-lock.json web-ext-artifacts node_modules README.md .gitignore vitest.config.js \"test/**\" --firefox=firefox",
-    "build": "web-ext build --source-dir=. --no-config-discovery --ignore-files package.json package-lock.json web-ext-artifacts node_modules README.md .gitignore vitest.config.js \"test/**\" --overwrite-dest",
-    "sign":  "web-ext sign --source-dir=. --no-config-discovery --ignore-files package.json package-lock.json web-ext-artifacts node_modules README.md .gitignore vitest.config.js \"test/**\" --channel=unlisted --api-key=$WEB_EXT_API_KEY --api-secret=$WEB_EXT_API_SECRET",
+    "lint":  "set -f; web-ext lint --source-dir=. --no-config-discovery --ignore-files $(sh scripts/packaging.sh ignore)",
+    "start": "set -f; web-ext run --source-dir=. --no-config-discovery --ignore-files $(sh scripts/packaging.sh ignore) --firefox=firefox",
+    "build": "set -f; web-ext build --source-dir=. --no-config-discovery --ignore-files $(sh scripts/packaging.sh ignore) --overwrite-dest",
+    "sign":  "set -f; web-ext sign --source-dir=. --no-config-discovery --ignore-files $(sh scripts/packaging.sh ignore) --channel=unlisted --api-key=$WEB_EXT_API_KEY --api-secret=$WEB_EXT_API_SECRET",
     "test:unit": "vitest run"
   },
   "devDependencies": {