ci(release): split tag creation and release into separate workflows

- Add `create-tag.yml`: creates annotated tag at a specified commit or
  latest main HEAD, with duplicate tag and commit validation
- Simplify `release.yml`: only accepts existing tags, removes create_tag
  toggle, validates tag via GitHub API before checkout
- Always checkout main branch (fetch-depth: 0 fetches full history),
  then create tag at the specified commit

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/create-tag.yml

@@ -0,0 +1,60 @@
+name: Create Tag
+
+on:
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: "Tag name (required, e.g. v0.2.0)"
+        required: true
+        type: string
+      commit:
+        description: "Target commit SHA (leave empty for latest main)"
+        required: false
+        type: string
+        default: ""
+
+jobs:
+  create-tag:
+    name: Create Git Tag
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+          ref: main
+
+      - name: Validate commit exists
+        if: ${{ inputs.commit != '' }}
+        shell: bash
+        run: |
+          if ! git cat-file -t "${{ inputs.commit }}" &>/dev/null; then
+            echo "::error::Commit '${{ inputs.commit }}' does not exist."
+            exit 1
+          fi
+
+      - name: Check tag does not already exist
+        shell: bash
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          if gh api "repos/${{ github.repository }}/git/ref/tags/${{ inputs.tag }}" --silent 2>/dev/null; then
+            echo "::error::Tag '${{ inputs.tag }}' already exists."
+            exit 1
+          fi
+
+      - name: Create and push tag
+        shell: bash
+        run: |
+          TARGET="${{ inputs.commit || 'HEAD' }}"
+          COMMIT_SHA=$(git rev-parse "$TARGET")
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git tag -a "${{ inputs.tag }}" "$COMMIT_SHA" -m "Release ${{ inputs.tag }}"
+          git push origin "${{ inputs.tag }}"
+          echo "### Tag Created" >> "$GITHUB_STEP_SUMMARY"
+          echo "- **Tag:** \`${{ inputs.tag }}\`" >> "$GITHUB_STEP_SUMMARY"
+          echo "- **Commit:** \`${COMMIT_SHA}\`" >> "$GITHUB_STEP_SUMMARY"
+          echo "- **Branch:** \`$(git branch -r --contains "$COMMIT_SHA" | head -1 | xargs)\`" >> "$GITHUB_STEP_SUMMARY"

.github/workflows/release.yml

@@ -1,10 +1,10 @@
-name: Create Tag and Release
+name: Release
 
 on:
   workflow_dispatch:
     inputs:
       tag:
-        description: "Release tag (required, e.g. v0.2.0)"
+        description: "Existing tag to release (e.g. v0.2.0)"
         required: true
         type: string
       prerelease:
@@ -24,35 +24,23 @@ on:
         default: true
 
 jobs:
-  create-tag:
-    name: Create Git Tag
-    runs-on: ubuntu-latest
-    permissions:
-      contents: write
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v6
-        with:
-          fetch-depth: 0
-
-      - name: Create and push tag
-        shell: bash
-        env:
-          RELEASE_TAG: ${{ inputs.tag }}
-        run: |
-          git config user.name "github-actions[bot]"
-          git config user.email "github-actions[bot]@users.noreply.github.com"
-          git tag -a "$RELEASE_TAG" -m "Release $RELEASE_TAG"
-          git push origin "$RELEASE_TAG"
-
   release:
     name: GoReleaser Release
-    needs: create-tag
     runs-on: ubuntu-latest
     permissions:
       contents: write
       packages: write
     steps:
+      - name: Verify tag exists
+        shell: bash
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          if ! gh api "repos/${{ github.repository }}/git/ref/tags/${{ inputs.tag }}" --silent 2>/dev/null; then
+            echo "::error::Tag '${{ inputs.tag }}' does not exist. Create it first using the 'Create Tag' workflow."
+            exit 1
+          fi
+
       - name: Checkout tag
         uses: actions/checkout@v6
         with: