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