feat: add pi-only variant (pi without opencode) as basis for pi-devbox

All opencode-devbox variants set INSTALL_OPENCODE=true, so pointing pi-devbox
at with-pi dragged opencode along and made it ~a re-tag of latest-with-pi.
Add a 5th variant pi-only (INSTALL_OPENCODE=false, INSTALL_PI=true): pi +
companions (toolkit, extensions, fork, recall) + base tooling, no opencode
(~145 MB lighter than with-pi).

- Dockerfile.variant: document pi-only in the variant table.
- CI docker-publish-split.yml: new smoke-pi-only + build-variant-pi-only jobs
  (tags :VERSION-pi-only / :latest-pi-only, multi-arch); wired into
  promote-base-latest and update-description needs.
- validate.yml: new validate-pi-only main-branch gate job.
- smoke-test.sh: accept --variant pi-only; threshold 2750 MB; opencode-absent
  path already handled.
- Docs: HUB_TEMPLATE (regenerated DOCKER_HUB.md), README, AGENTS (variant/tag
  counts 4->5, 8->10 tags), .gitea/README, manual-host-publish.sh (5 variants),
  plan doc implementation note.

This is the single source of truth for joakimp/pi-devbox, which now FROMs
latest-pi-only. Versions unchanged (opencode 1.15.13, pi 0.78.0).

.gitea/workflows/docker-publish-split.yml

@@ -432,6 +432,53 @@ jobs:
           EXPECTED_OMOS_VERSION: ${{ needs.resolve-versions.outputs.omos_version }}
         run: bash scripts/smoke-test.sh opencode-devbox:smoke-omos-with-pi --variant omos-with-pi
 
+  smoke-pi-only:
+    needs: [base-decide, build-base, resolve-versions]
+    if: |
+      always() &&
+      needs.base-decide.result == 'success' &&
+      needs.resolve-versions.result == 'success' &&
+      (needs.build-base.result == 'success' || needs.build-base.result == 'skipped')
+    runs-on: ubuntu-latest
+    container:
+      image: catthehacker/ubuntu:act-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: echo 'precedence ::ffff:0:0/96  100' >> /etc/gai.conf
+      - run: |
+          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
+          docker system prune -af --volumes || true
+          docker builder prune -af || true
+      - uses: docker/setup-buildx-action@v4
+        with: {driver-opts: network=host}
+      - uses: docker/login-action@v3
+        with:
+          username: ${{ vars.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      - uses: docker/build-push-action@v7
+        with:
+          context: .
+          file: Dockerfile.variant
+          platforms: linux/amd64
+          push: false
+          load: true
+          tags: opencode-devbox:smoke-pi-only
+          build-args: |
+            BASE_IMAGE=${{ env.IMAGE }}:${{ needs.base-decide.outputs.base_tag }}
+            INSTALL_OPENCODE=false
+            INSTALL_OMOS=false
+            INSTALL_PI=true
+            PI_VERSION=${{ needs.resolve-versions.outputs.pi_version }}
+            PI_FORK_REF=${{ needs.resolve-versions.outputs.fork_ref }}
+            PI_OBSMEM_REF=${{ needs.resolve-versions.outputs.obsmem_ref }}
+      - env:
+          EXPECTED_PI_VERSION: ${{ needs.resolve-versions.outputs.pi_version }}
+        run: bash scripts/smoke-test.sh opencode-devbox:smoke-pi-only --variant pi-only
+
   # ── Phase 4: multi-arch publish per variant ────────────────────────
 
   build-variant-base:
@@ -727,6 +774,81 @@ jobs:
           echo "==> All 3 build+push attempts failed"
           exit 1
 
+  build-variant-pi-only:
+    needs: [base-decide, smoke-pi-only, resolve-versions]
+    runs-on: ubuntu-latest
+    container:
+      image: catthehacker/ubuntu:act-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: echo 'precedence ::ffff:0:0/96  100' >> /etc/gai.conf
+      - run: |
+          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
+          docker system prune -af --volumes || true
+          docker builder prune -af || true
+      - uses: docker/setup-qemu-action@v3
+        with: {platforms: arm64}
+      - uses: docker/setup-buildx-action@v4
+        with: {driver-opts: network=host}
+      - uses: docker/login-action@v3
+        with:
+          username: ${{ vars.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      - name: Compute version-specific tags
+        id: tags
+        run: |
+          VERSION="${{ env.RELEASE_TAG }}"
+          { echo "tags<<EOF"
+            echo "${IMAGE}:${VERSION}-pi-only"
+            if [ "${{ env.PROMOTE_LATEST }}" = "true" ]; then
+              echo "${IMAGE}:latest-pi-only"
+            fi
+            echo "EOF"
+          } >> "$GITHUB_OUTPUT"
+      - name: Build and push variant (with retry)
+        shell: bash
+        env:
+          TAGS: ${{ steps.tags.outputs.tags }}
+          BASE_IMAGE_FULL: ${{ env.IMAGE }}:${{ needs.base-decide.outputs.base_tag }}
+          PI_VERSION: ${{ needs.resolve-versions.outputs.pi_version }}
+          FORK_REF: ${{ needs.resolve-versions.outputs.fork_ref }}
+          OBSMEM_REF: ${{ needs.resolve-versions.outputs.obsmem_ref }}
+        run: |
+          set -euo pipefail
+          TAG_FLAGS=()
+          while IFS= read -r t; do [[ -n "$t" ]] && TAG_FLAGS+=( -t "$t" ); done <<< "${TAGS}"
+          # 3-attempt retry (see build-base step for rationale). Variant: pi-only.
+          for attempt in 1 2 3; do
+            echo "==> Build+push attempt ${attempt}/3"
+            if docker buildx build \
+                --platform linux/amd64,linux/arm64 \
+                --file Dockerfile.variant \
+                --push \
+                --build-arg "BASE_IMAGE=${BASE_IMAGE_FULL}" \
+                --build-arg "INSTALL_OPENCODE=false" \
+                --build-arg "INSTALL_OMOS=false" \
+                --build-arg "INSTALL_PI=true" \
+                --build-arg "PI_VERSION=${PI_VERSION}" \
+                --build-arg "PI_FORK_REF=${FORK_REF}" \
+                --build-arg "PI_OBSMEM_REF=${OBSMEM_REF}" \
+                "${TAG_FLAGS[@]}" \
+                .; then
+              echo "==> Attempt ${attempt} succeeded"
+              exit 0
+            fi
+            if [[ "${attempt}" -lt 3 ]]; then
+              backoff=$(( attempt * 15 ))
+              echo "==> Attempt ${attempt} failed, sleeping ${backoff}s before retry"
+              sleep "${backoff}"
+            fi
+          done
+          echo "==> All 3 build+push attempts failed"
+          exit 1
+
   # ── Phase 5: promote base-<hash> → base-latest (manifest copy only) ─
   promote-base-latest:
     needs:
@@ -735,6 +857,7 @@ jobs:
       - build-variant-omos
       - build-variant-with-pi
       - build-variant-omos-with-pi
+      - build-variant-pi-only
     # Skip on cache-hit base builds: when need_build=false, base-latest
     # already points at the same digest as base-<hash>, so the retag is
     # a tautology and any transient failure of it is purely cosmetic.
@@ -787,6 +910,7 @@ jobs:
       - build-variant-omos
       - build-variant-with-pi
       - build-variant-omos-with-pi
+      - build-variant-pi-only
     # Run when at least the base variant published — don't let a single
     # variant failure (e.g., omos-with-pi smoke threshold) prevent Hub
     # description refresh for the other variants that did publish.