fix(ci): every publishing job builds the commit that fired the run, not the branch tip (#4427)

BUILD_REF resolved to the branch name on ordinary runs, and each job's
checkout re-resolves a branch when that job starts. A push that lands
mid-run therefore moved the later jobs onto the new tip:

- run 7499 signed 423275a's extension;
- its build-web then checked out 83e1382, derived a version nobody had
  signed, and got a 404 on the download.

The guard failed closed. A job without such a guard would have published
a commit the run's lanes never tested.

The lanes already check out github.sha by default. BUILD_REF is now
github.sha on every trigger except the base refresh, which keeps `main`
and its branch guard. The publish therefore builds exactly what the
lanes passed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

.forgejo/workflows/build.yml

@@ -142,6 +142,16 @@ concurrency:
 # Deriving it per job invites the two halves to disagree: sign-extension would
 # derive dev's extension version while build-web bundled main's, and the
 # release download would 404 on a version that exists perfectly well.
+#
+# On every other trigger it is the COMMIT that fired (`github.sha`), never the
+# branch name. A branch is re-resolved by each job's checkout when that job
+# starts, so a push landing mid-run moved the later jobs onto the new tip: run
+# 7499 signed 423275a's extension, then build-web checked out 83e1382 (pushed
+# while 7499 ran), derived a version nobody had signed, and 404'd on the
+# download (#4427). The guard failed closed that time; a job without one would
+# have published a commit the run's own lanes never tested — the lanes check
+# out `github.sha` by default, so pinning here makes the publish build exactly
+# what they passed.
 # IS THIS A BASE REFRESH? Asked in five places and previously spelled five
 # ways — `github.event_name == 'schedule'` in an `if:`, `$GITHUB_EVENT_NAME` in
 # one shell, an `EVENT:` env passed into another, and a bare expression on
@@ -178,7 +188,7 @@ concurrency:
 # where a step-level `if:` needs the answer before any shell runs.
 env:
   IS_REFRESH: ${{ (github.event_name == 'schedule' || format('{0}', github.event.inputs.refresh) == 'true') && 'true' || 'false' }}
-  BUILD_REF: ${{ (github.event_name == 'schedule' || format('{0}', github.event.inputs.refresh) == 'true') && 'main' || github.ref }}
+  BUILD_REF: ${{ (github.event_name == 'schedule' || format('{0}', github.event.inputs.refresh) == 'true') && 'main' || github.sha }}
 
 # Requires repo secret RELEASE_TOKEN — a Forgejo PAT with scopes:
 #   - write:package, read:package  (for docker push to git.fabledsword.com)
@@ -605,8 +615,8 @@ jobs:
       - uses: actions/checkout@v4
         with:
           # Not the triggering ref — see the `env:` block at the top. On a
-          # scheduled refresh this is `main`; on everything else it is the ref
-          # that fired, so this is a no-op on every ordinary path.
+          # scheduled refresh this is `main`; on everything else it is the
+          # commit that fired, the same one the lanes above tested.
           ref: ${{ env.BUILD_REF }}
           # Full history is load-bearing, not a convenience: the version this
           # job signs is derived from the commit TIME of the newest packaged
@@ -945,8 +955,8 @@ jobs:
       - uses: actions/checkout@v4
         with:
           # Not the triggering ref — see the `env:` block at the top. On a
-          # scheduled refresh this is `main`; on everything else it is the ref
-          # that fired, so this is a no-op on every ordinary path.
+          # scheduled refresh this is `main`; on everything else it is the
+          # commit that fired, the same one the lanes above tested.
           ref: ${{ env.BUILD_REF }}
           # Full history: this job RE-DERIVES the extension version rather than
           # being handed it, and a depth-1 clone derives a wrong, too-low value
@@ -2174,8 +2184,8 @@ jobs:
       - uses: actions/checkout@v4
         with:
           # Not the triggering ref — see the `env:` block at the top. On a
-          # scheduled refresh this is `main`; on everything else it is the ref
-          # that fired, so this is a no-op on every ordinary path.
+          # scheduled refresh this is `main`; on everything else it is the
+          # commit that fired, the same one the lanes above tested.
           ref: ${{ env.BUILD_REF }}
           # Full history: this job derives its artifact's version from the
           # commit its shipped files last changed in (milestone 313). A