Cut v1.15.12 — revert v4.0.0 pin (busted), bump pi to 0.76.0

The v1.15.11b experiment confirmed setup-buildx-action@v4.1.0 is NOT
the regressor: pinning all 9 references to @v4.0.0 reproduced the
exact same '400 Bad request' from registry-1.docker.io on the first
layer-blob PUT. CI run #336 failed twice (original + Gitea auto-rerun),
both with HTML 400 bodies (CDN-tier rejection) at Offset:0. UUIDs and
_state signatures differ across attempts; only the failure pattern is
stable.

Reverting all 9 pins back to @v4 — keeping a wrong pin holds us off
action improvements with no benefit. Real suspects now narrow to:
runner-image (catthehacker:act-latest, floating), runner-2 host
network egress, buildx 0.34.x signed _state token format, or per-repo
Hub-side state. Investigation deferred; this release ships via manual
docker buildx build --push from a developer Orbstack to bypass the
broken runner-network → Hub-CDN combo (we know that path works in
~25s for the same multi-arch build to the same Hub account).

PI_VERSION=latest resolves to pi-coding-agent 0.76.0 (published
2026-05-27 20:03 UTC). OPENCODE_VERSION stays at 1.15.11 (no upstream
bump since 1.15.11 was published 2026-05-27 03:59 UTC).

Files:
- .gitea/workflows/docker-publish-split.yml: 9 setup-buildx-action
  references reverted from @v4.0.0 to @v4
- CHANGELOG.md: v1.15.12 entry with regression triage status
  (ruled-out vs still-suspect)

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

@@ -174,7 +174,7 @@ jobs:
           platforms: arm64
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v4.0.0
+        uses: docker/setup-buildx-action@v4
         with:
           driver-opts: network=host
 
@@ -223,7 +223,7 @@ jobs:
             /usr/lib/jvm 2>/dev/null || true
           docker system prune -af --volumes || true
           docker builder prune -af || true
-      - uses: docker/setup-buildx-action@v4.0.0
+      - uses: docker/setup-buildx-action@v4
         with: {driver-opts: network=host}
       - uses: docker/login-action@v3
         with:
@@ -267,7 +267,7 @@ jobs:
             /usr/lib/jvm 2>/dev/null || true
           docker system prune -af --volumes || true
           docker builder prune -af || true
-      - uses: docker/setup-buildx-action@v4.0.0
+      - uses: docker/setup-buildx-action@v4
         with: {driver-opts: network=host}
       - uses: docker/login-action@v3
         with:
@@ -312,7 +312,7 @@ jobs:
             /usr/lib/jvm 2>/dev/null || true
           docker system prune -af --volumes || true
           docker builder prune -af || true
-      - uses: docker/setup-buildx-action@v4.0.0
+      - uses: docker/setup-buildx-action@v4
         with: {driver-opts: network=host}
       - uses: docker/login-action@v3
         with:
@@ -357,7 +357,7 @@ jobs:
             /usr/lib/jvm 2>/dev/null || true
           docker system prune -af --volumes || true
           docker builder prune -af || true
-      - uses: docker/setup-buildx-action@v4.0.0
+      - uses: docker/setup-buildx-action@v4
         with: {driver-opts: network=host}
       - uses: docker/login-action@v3
         with:
@@ -403,7 +403,7 @@ jobs:
           docker builder prune -af || true
       - uses: docker/setup-qemu-action@v3
         with: {platforms: arm64}
-      - uses: docker/setup-buildx-action@v4.0.0
+      - uses: docker/setup-buildx-action@v4
         with: {driver-opts: network=host}
       - uses: docker/login-action@v3
         with:
@@ -451,7 +451,7 @@ jobs:
           docker builder prune -af || true
       - uses: docker/setup-qemu-action@v3
         with: {platforms: arm64}
-      - uses: docker/setup-buildx-action@v4.0.0
+      - uses: docker/setup-buildx-action@v4
         with: {driver-opts: network=host}
       - uses: docker/login-action@v3
         with:
@@ -500,7 +500,7 @@ jobs:
           docker builder prune -af || true
       - uses: docker/setup-qemu-action@v3
         with: {platforms: arm64}
-      - uses: docker/setup-buildx-action@v4.0.0
+      - uses: docker/setup-buildx-action@v4
         with: {driver-opts: network=host}
       - uses: docker/login-action@v3
         with:
@@ -549,7 +549,7 @@ jobs:
           docker builder prune -af || true
       - uses: docker/setup-qemu-action@v3
         with: {platforms: arm64}
-      - uses: docker/setup-buildx-action@v4.0.0
+      - uses: docker/setup-buildx-action@v4
         with: {driver-opts: network=host}
       - uses: docker/login-action@v3
         with: