feat: add zip packaging target and woodpecker release pipeline

- Makefile gains `make package` target that builds and zips the provider
  binary in the format expected by artifactapi's local terraform registry:
  terraform-provider-artifactapi_{version}_{os}_{arch}.zip
- New .woodpecker/release.yml pipeline triggers on tag push, builds the
  zip, and uploads it to artifactapi via the local files API
- Secrets: artifactapi_url and artifactapi_repo

.woodpecker/release.yml

@@ -0,0 +1,21 @@
+when:
+  - event: tag
+
+steps:
+  - name: package
+    image: golang:1.25
+    commands:
+      - apt-get update && apt-get install -y zip
+      - make package VERSION=${CI_COMMIT_TAG}
+
+  - name: upload
+    image: curlimages/curl:latest
+    secrets: [artifactapi_url, artifactapi_repo]
+    commands:
+      - |
+        VERSION=$(echo ${CI_COMMIT_TAG} | sed 's/^v//')
+        FILE="terraform-provider-artifactapi_${VERSION}_linux_amd64.zip"
+        curl -f -X PUT \
+          "$${ARTIFACTAPI_URL}/api/v2/remotes/$${ARTIFACTAPI_REPO}/files/unkin/artifactapi/$${FILE}" \
+          -H "Content-Type: application/zip" \
+          --data-binary @"$${FILE}"