fix: make prepare action resilient for reruns

- Disable setup-go module cache path in prepare action's source-run mode
  to avoid invalid '..' cache-dependency-path patterns.
- Make commit/tag step idempotent when release tag already exists locally
  or remotely.
- Skip empty commit attempts when no release files changed while still
  allowing first-time tag creation.

prepare/action.yml

@@ -109,8 +109,7 @@ runs:
       uses: actions/setup-go@v5
       with:
         go-version: '1.26.1'
-        cache: true
-        cache-dependency-path: ${{ github.action_path }}/../go.sum
+        cache: false
 
     - name: Restore cached vociferate binary
       id: cache-vociferate
@@ -192,11 +191,6 @@ runs:
       run: |
         set -euo pipefail
 
-        if git rev-parse "$RELEASE_TAG" >/dev/null 2>&1; then
-          echo "Tag ${RELEASE_TAG} already exists" >&2
-          exit 1
-        fi
-
         case "$GITHUB_SERVER_URL" in
           https://*)
             authed_remote="https://oauth2:${TOKEN}@${GITHUB_SERVER_URL#https://}/${GITHUB_REPOSITORY}.git"
@@ -214,11 +208,21 @@ runs:
         git config user.email "$GIT_USER_EMAIL"
         git remote set-url origin "$authed_remote"
 
+        if git rev-parse "$RELEASE_TAG" >/dev/null 2>&1 || git ls-remote --exit-code --tags origin "refs/tags/${RELEASE_TAG}" >/dev/null 2>&1; then
+          echo "Tag ${RELEASE_TAG} already exists; skipping commit/tag/push."
+          exit 0
+        fi
+
         for f in $GIT_ADD_FILES; do
           git add "$f"
         done
 
+        if git diff --cached --quiet; then
+          echo "No staged release file changes; tagging current HEAD as ${RELEASE_TAG}."
+        else
           git commit -m "release: prepare ${RELEASE_TAG}"
+        fi
+
         git tag "$RELEASE_TAG"
         git push origin HEAD
         git push origin "$RELEASE_TAG"