Reclaim runner disk before load: true smoke builds

v1.14.31 publish and validate both hit 'No space left on device' on
single-arch amd64 smoke/validate builds. The image has crossed ~3 GB
and the runner's ~40 GB overlay starts ~70% full, so 'load: true'
peak disk (tarball + unpacked image + buildx cache) no longer fits.

Add a 'Reclaim runner disk' step to validate-base, validate-omos,
smoke-base, smoke-omos. Strips catthehacker-resident toolchains we
never use (hosted-tool-cache, dotnet, android, powershell, swift,
ghc, jvm, microsoft, chromium, boost), then runs 'docker system
prune -af --volumes' + 'docker builder prune -af' against the
runner's dockerd before setup-buildx-action. Expected reclaim is
6-12 GB depending on what's resident.

Deliberately NOT in the per-arch matrix build jobs — push-by-digest
doesn't need it and pruning in parallel jobs risks one job nuking
another's in-flight buildx cache. Also add workflow-level
concurrency on docker-publish.yml so concurrent tag pushes serialize
cleanly.

.gitea/workflows/validate.yml

@@ -46,6 +46,34 @@ jobs:
         run: |
           echo 'precedence ::ffff:0:0/96  100' >> /etc/gai.conf
 
+      # The runner's overlay disk starts ~70% full. `load: true` peak disk
+      # is tarball + unpacked image + buildx cache, which tips it over
+      # once the image crosses ~3 GB. Strip catthehacker-resident
+      # toolchains we never use and any stale docker state up front.
+      - name: Reclaim runner disk
+        run: |
+          set -x
+          df -h / || true
+          rm -rf \
+            /opt/hostedtoolcache \
+            /opt/microsoft \
+            /opt/az \
+            /opt/ghc \
+            /usr/local/.ghcup \
+            /usr/share/dotnet \
+            /usr/share/swift \
+            /usr/local/lib/android \
+            /usr/local/share/powershell \
+            /usr/local/share/chromium \
+            /usr/local/share/boost \
+            /usr/lib/jvm 2>/dev/null || true
+          apt-get clean || true
+          rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* || true
+          docker system df || true
+          docker system prune -af --volumes || true
+          docker builder prune -af || true
+          df -h / || true
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
         with:
@@ -76,6 +104,30 @@ jobs:
         run: |
           echo 'precedence ::ffff:0:0/96  100' >> /etc/gai.conf
 
+      - name: Reclaim runner disk
+        run: |
+          set -x
+          df -h / || true
+          rm -rf \
+            /opt/hostedtoolcache \
+            /opt/microsoft \
+            /opt/az \
+            /opt/ghc \
+            /usr/local/.ghcup \
+            /usr/share/dotnet \
+            /usr/share/swift \
+            /usr/local/lib/android \
+            /usr/local/share/powershell \
+            /usr/local/share/chromium \
+            /usr/local/share/boost \
+            /usr/lib/jvm 2>/dev/null || true
+          apt-get clean || true
+          rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* || true
+          docker system df || true
+          docker system prune -af --volumes || true
+          docker builder prune -af || true
+          df -h / || true
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
         with: