fix(review): remove expression syntax from a run-block comment

Author: zfarrell

.github/workflows/claude-pr-review.yml

@@ -257,8 +257,16 @@ jobs:
             echo "${DELIMITER}"
           } >> $GITHUB_OUTPUT
 
-          # Title and body reach the shell through env, never a ${{ }} interpolation: both
-          # are attacker-controlled text and would otherwise be spliced into this script.
+          # Title and body reach the shell through env, never an Actions expression
+          # interpolation: both are attacker-controlled text and would otherwise be spliced
+          # into this script.
+          #
+          # That expression syntax cannot be written out inside this run block, not even in a
+          # comment. Actions parses those delimiters in the block's *string value*, comments
+          # included, and an empty pair is a syntax error that makes the whole workflow
+          # unparseable -- no jobs, no required check, every PR in the org blocked behind
+          # "Please close and reopen the PR to trigger this workflow". A YAML comment outside
+          # a block scalar is safe, because the YAML parser strips it before Actions looks.
           # First in the file on purpose: the byte cap keeps the head, so anything the
           # reviewer must not miss has to be above the blocks that can grow.
           if [ -s "$WARN_FILE" ]; then