fix(release): use case-insensitive label matching in check_release (#158)

## What/Why

The old GitHub Actions `contains()` expression was case-insensitive, but
the bash replacement introduced in #156 compared labels case-sensitively.
Lowercase labels with `${PR_LABELS,,}` to preserve the original semantics.

## Proof it works

actionlint and shellcheck both pass with no errors.

## Risk + AI role

Low -- one-liner fix. AI-assisted (Claude Opus 4.6).

## Review focus

Confirm `${PR_LABELS,,}` (bash 4+ lowercase expansion) works on the
ubuntu-latest runner.

Signed-off-by: jmeridth <jmeridth@gmail.com>

Author: jmeridth

.github/workflows/release.yaml

@@ -106,7 +106,7 @@ jobs:
           elif [ "$PR_MERGED" != "true" ]; then
             echo "PR not merged: skipping release"
           else
-            IFS=',' read -ra labels <<< "$PR_LABELS"
+            IFS=',' read -ra labels <<< "${PR_LABELS,,}"
             if [ "$RELEASE_ONLY" = "true" ]; then
               for label in "${labels[@]}"; do
                 if [ "$label" = "release" ]; then