fix(release): partition CGO split builds by target to stop cross-arch failure

GoReleaser Pro --split defaults to partitioning by GOOS only, so each
linux matrix runner built both linux/amd64 and linux/arm64. With
CGO_ENABLED=1 (go-tree-sitter), the native gcc cannot assemble the other
arch's CGO runtime (gcc_arm64.S: no such instruction), failing the build.

- .goreleaser.yaml: set partial.by=target (partition by GOOS+GOARCH)
- workflow-prepare.sh: export GGOOS/GGOARCH from the matrix TARGET_* vars
  so each native runner builds exactly its own target

GGOOS/GGOARCH are filter-only and do not bleed into before-hooks, unlike
GOOS/GOARCH.

.goreleaser.yaml

@@ -5,6 +5,15 @@ version: 2
 
 project_name: mcp-filepuff
 
+# Split CGO builds by full target (GOOS+GOARCH), not just GOOS.
+# Each release matrix runner builds natively for exactly one target via the
+# GGOOS/GGOARCH filters set in workflow-prepare.sh. Without `by: target` the
+# split partitions by GOOS only, so a single linux runner would try to build
+# both amd64 and arm64 — and native gcc cannot assemble the other arch's CGO
+# runtime (gcc_arm64.S errors), breaking the release.
+partial:
+  by: target
+
 before:
   hooks:
     - go mod tidy

workflow-prepare.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# Invoked by lukaszraczylo/shared-actions go-release-cgo.yaml before GoReleaser,
+# with TARGET_GOOS/TARGET_GOARCH set from the build matrix entry.
+#
+# GoReleaser Pro `release --split` partitions builds across matrix runners.
+# Its default partition is by GOOS only, so every linux runner would build
+# BOTH linux/amd64 and linux/arm64. With CGO_ENABLED=1 (go-tree-sitter needs
+# CGO) the native gcc on one runner cannot assemble the other arch's CGO
+# runtime (`gcc_arm64.S: no such instruction`), failing the release.
+#
+# Exporting GGOOS/GGOARCH restricts each runner to its own target. These are
+# filter-only vars (unlike GOOS/GOARCH, which bleed into before-hooks such as
+# `go run`/`go generate`) and require `partial.by: target` in .goreleaser.yaml.
+set -euo pipefail
+
+if [ -n "${GITHUB_ENV:-}" ] && [ -n "${TARGET_GOOS:-}" ] && [ -n "${TARGET_GOARCH:-}" ]; then
+  echo "GGOOS=${TARGET_GOOS}" >>"${GITHUB_ENV}"
+  echo "GGOARCH=${TARGET_GOARCH}" >>"${GITHUB_ENV}"
+  echo "Pinned GoReleaser split target: GGOOS=${TARGET_GOOS} GGOARCH=${TARGET_GOARCH}"
+fi