Add release tag validation workflow

.woodpecker/release.yml

@@ -0,0 +1,33 @@
+when:
+  - event: tag
+
+steps:
+  - name: validate-release
+    image: golang:1.25.5
+    commands:
+      - |
+        set -eu
+
+        version="$CI_COMMIT_TAG"
+        release_note="docs/releases/$version.md"
+
+        if ! printf '%s\n' "$version" | grep -E -x 'v(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)' >/dev/null; then
+          printf '%s\n' "invalid release tag: $version" >&2
+          exit 1
+        fi
+        if [ ! -s "$release_note" ]; then
+          printf '%s\n' "missing release note: $release_note" >&2
+          exit 1
+        fi
+        if ! grep -F -x "# Notarius $version" "$release_note" >/dev/null; then
+          printf '%s\n' "release note heading does not match $version" >&2
+          exit 1
+        fi
+        for heading in '## Summary' '## Compatibility' '## Upgrade' '## Changes'; do
+          if ! grep -F -x "$heading" "$release_note" >/dev/null; then
+            printf '%s\n' "release note is missing heading: $heading" >&2
+            exit 1
+          fi
+        done
+
+        ./scripts/check-release-source.sh "$version"

docs/roadmap/implementation.md

@@ -192,7 +192,7 @@ git status --short
   matrix.
 - Successful execution leaves the worktree and index unchanged.
 
-## Stage 3: Add Validation-Only Tag CI
+## Stage 3: Add Validation-Only Tag CI ✅
 
 ### Goal