fix(ci): POSIX-safe SHORT_SHA in build.yml (runner uses dash, not bash)

`${GITHUB_SHA:0:7}` substring expansion is bash-only; the runner
executes the step via dash/BusyBox sh and errored with
`Bad substitution` (act_runner workflow shell, observed on the
65386f0 main-push run). Switched to
`SHORT_SHA=$(printf '%s' "$GITHUB_SHA" | cut -c1-7)` which works in
both shells. Both build-web and build-ml tag-determination steps
updated.

.forgejo/workflows/build.yml

@@ -258,7 +258,11 @@ jobs:
           #   anything else  → safety net; shouldn't fire given the `on:`
           #                    config above. Tag :dev to surface the
           #                    unexpected run in the registry.
-          SHORT_SHA="${GITHUB_SHA:0:7}"
+          # POSIX-safe substring (the runner shell is dash/BusyBox sh, not
+          # bash — `${var:0:7}` errors with "Bad substitution"; cut works
+          # everywhere). Operator-flagged 2026-06-01 after first :c-<sha>
+          # main-push build failed at this step.
+          SHORT_SHA=$(printf '%s' "$GITHUB_SHA" | cut -c1-7)
           if [ "${GITHUB_REF#refs/tags/}" != "${GITHUB_REF}" ]; then
             TAG_NAME="${GITHUB_REF#refs/tags/}"
             echo "tags=git.fabledsword.com/bvandeusen/fabledcurator:${TAG_NAME}" >> "$GITHUB_OUTPUT"
@@ -297,7 +301,11 @@ jobs:
           # safety-net dev) including the per-commit :c-<short_sha> tag
           # on main-push per the family release-posture rule. The -ml
           # image follows the same release cadence as the web image.
-          SHORT_SHA="${GITHUB_SHA:0:7}"
+          # POSIX-safe substring (the runner shell is dash/BusyBox sh, not
+          # bash — `${var:0:7}` errors with "Bad substitution"; cut works
+          # everywhere). Operator-flagged 2026-06-01 after first :c-<sha>
+          # main-push build failed at this step.
+          SHORT_SHA=$(printf '%s' "$GITHUB_SHA" | cut -c1-7)
           if [ "${GITHUB_REF#refs/tags/}" != "${GITHUB_REF}" ]; then
             TAG_NAME="${GITHUB_REF#refs/tags/}"
             echo "tags=git.fabledsword.com/bvandeusen/fabledcurator-ml:${TAG_NAME}" >> "$GITHUB_OUTPUT"