fix: extract decorate-pr comment rendering from action yaml

decorate-pr/build-comment.sh

@@ -0,0 +1,71 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+tmp_file=$(mktemp)
+trap 'rm -f "$tmp_file"' EXIT
+
+{
+  printf '%s\n' '<!-- vociferate-pr-review -->'
+  printf '\n## %s\n\n' "$COMMENT_TITLE"
+  printf '### Coverage\n'
+  printf '![Coverage Badge](%s)\n\n' "$BADGE_URL"
+  printf '**Coverage:** %s%%\n' "$COVERAGE_PCT"
+} > "$tmp_file"
+
+if [[ "$GATE_ENABLED" == "true" ]]; then
+  gate_status="Pass"
+  if [[ "$GATE_PASSED" != "true" ]]; then
+    if [[ "$GATE_MODE" == "strict" ]]; then
+      gate_status="Fail"
+    else
+      gate_status="Warning"
+    fi
+  fi
+
+  {
+    printf '\n### Changelog Gate\n'
+    printf '**Status:** %s\n\n' "$gate_status"
+  } >> "$tmp_file"
+
+  if [[ "$DOCS_ONLY" == "true" ]]; then
+    printf '%s\n\n' 'This PR only modifies documentation; changelog entry not required.' >> "$tmp_file"
+  elif [[ "$GATE_PASSED" == "true" ]]; then
+    printf 'Found %s line(s) added to Unreleased section.\n\n' "$ADDITIONS_COUNT" >> "$tmp_file"
+  else
+    printf '**Issue:** %s\n\n' "$FAILURE_REASON" >> "$tmp_file"
+    cat >> "$tmp_file" <<'EOF'
+**How to fix:** Add an entry under the appropriate subsection in the `## [Unreleased]` section of `CHANGELOG.md`. Use one of:
+- `### Breaking` for backwards-incompatible changes
+- `### Added` for new features
+- `### Changed` for behavior changes
+- `### Removed` for deprecated removals
+- `### Fixed` for bug fixes
+
+Example:
+```markdown
+## [Unreleased]
+### Added
+- New changelog gate validation for PR decoration.
+```
+
+EOF
+  fi
+fi
+
+if [[ -n "$UNRELEASED" ]]; then
+  {
+    printf '### Unreleased Changes\n\n'
+    printf '%s\n\n' "$UNRELEASED"
+  } >> "$tmp_file"
+fi
+
+cat >> "$tmp_file" <<'EOF'
+---
+*This comment was automatically generated by [vociferate/decorate-pr](https://git.hrafn.xyz/aether/vociferate).*
+EOF
+
+delimiter="EOF_COMMENT"
+printf 'comment_body<<%s\n' "$delimiter" >> "$GITHUB_OUTPUT"
+cat "$tmp_file" >> "$GITHUB_OUTPUT"
+printf '%s\n' "$delimiter" >> "$GITHUB_OUTPUT"