ci: add nightly snapshot build (02:00 UTC schedule)

Scheduled runs use -SNAPSHOT version suffix and publish to
starshipos-snapshots Nexus repo instead of starshipos-releases.
Push-to-master releases continue to use -<sha> suffix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -5,9 +5,12 @@ on:
     branches: [master]
   pull_request:
     branches: [master]
+  schedule:
+    - cron: '0 2 * * *'   # 02:00 UTC nightly snapshot
 
 env:
-  NEXUS: https://nexus.starshipos-f18.org/repository/starshipos-releases
+  NEXUS:      https://nexus.starshipos-f18.org/repository/starshipos-releases
+  NEXUS_SNAP: https://nexus.starshipos-f18.org/repository/starshipos-snapshots
 
 jobs:
   build:
@@ -68,9 +71,15 @@ jobs:
           LITHOS=$(grep 'LITHOS_VERSION\s*?=' Makefile.starkernel | head -1 | sed 's/.*= *//')
           SF=$(grep '^VERSION\s*?=' Makefile | head -1 | sed 's/.*= *//')
           SHA=$(git rev-parse --short HEAD)
-          echo "lithos=${LITHOS}-${SHA}" >> $GITHUB_OUTPUT
-          echo "sf=${SF}-${SHA}"         >> $GITHUB_OUTPUT
-          echo "sha=${SHA}"              >> $GITHUB_OUTPUT
+          if [ "${{ github.event_name }}" = "schedule" ]; then
+            SUFFIX="-SNAPSHOT"
+          else
+            SUFFIX="-${SHA}"
+          fi
+          echo "lithos=${LITHOS}${SUFFIX}" >> $GITHUB_OUTPUT
+          echo "sf=${SF}${SUFFIX}"         >> $GITHUB_OUTPUT
+          echo "sha=${SHA}"                >> $GITHUB_OUTPUT
+          echo "is_snapshot=${{ github.event_name == 'schedule' }}" >> $GITHUB_OUTPUT
 
       - name: Build StarForth VM
         run: make T=${{ matrix.board }}
@@ -130,7 +139,7 @@ jobs:
             build/${{ matrix.vm_arch }}/fastest/starforth
 
       - name: Publish to Nexus
-        if: github.ref == 'refs/heads/master'
+        if: github.ref == 'refs/heads/master' || github.event_name == 'schedule'
         env:
           NEXUS_PASS: ${{ secrets.NEXUS_CI_PASSWORD }}
           BOARD: ${{ matrix.board }}
@@ -139,15 +148,18 @@ jobs:
           KVER: ${{ steps.ver.outputs.lithos }}
           SFVER: ${{ steps.ver.outputs.sf }}
           IMG_TYPE: ${{ matrix.img_type }}
+          IS_SNAPSHOT: ${{ steps.ver.outputs.is_snapshot }}
         run: |
           set -e
+          REPO="${IS_SNAPSHOT:+$NEXUS_SNAP}"
+          REPO="${REPO:-$NEXUS}"
           push() {
             local coord="$1" file="$2"
             echo "→ org.f18:${coord}"
             curl -fsS --retry 3 \
               -u "ci-publisher:${NEXUS_PASS}" \
               -H "Content-Type: application/octet-stream" \
-              -X PUT "${NEXUS}/org/f18/${coord}" \
+              -X PUT "${REPO}/org/f18/${coord}" \
               --upload-file "$file"
           }