fix: address QA findings on detect-pipeline workflow

- Fix subdirectory matching: use prefix match for .github/* paths
  instead of exact dirname match (fixes .github/workflows/ not matching)
- Upgrade tj-actions/changed-files from v44 to v47 (Node 24 support)
- Extract detection logic into scripts/detect-pipeline.sh for testability
- Add 22 automated tests in scripts/test-detect-pipeline.sh covering
  infra-only, plugin code, mixed, and edge cases
- Add test-detection-logic CI job to run tests on every PR
- Update README.md to reference v47

cc @cpfarhood

Co-Authored-By: Paperclip <noreply@paperclip.ing>

scripts/detect-pipeline.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Reads a newline-separated list of changed files from stdin.
+# Outputs "pipeline-a" or "pipeline-b" to stdout.
+# Pipeline B: all files are infra-only (config, docs, workflows).
+# Pipeline A: any non-infra file present.
+
+detect_pipeline() {
+  local all_infra=true
+
+  while IFS= read -r file; do
+    [ -z "$file" ] && continue
+
+    local filename
+    local dir
+    filename=$(basename "$file")
+    dir=$(dirname "$file")
+
+    if [[ "$dir" == ".github" || "$dir" == .github/* ]] || \
+       [[ "$filename" == *.md ]] || \
+       [[ "$filename" == .eslintrc* ]] || \
+       [[ "$filename" == .prettierrc* ]] || \
+       [[ "$filename" == renovate.json* ]] || \
+       [[ "$filename" == .gitignore ]] || \
+       [[ "$filename" == .editorconfig ]] || \
+       [[ "$filename" == LICENSE ]]; then
+      continue
+    else
+      all_infra=false
+      break
+    fi
+  done
+
+  if [ "$all_infra" = true ]; then
+    echo "pipeline-b"
+  else
+    echo "pipeline-a"
+  fi
+}
+
+if [ "${BASH_SOURCE[0]}" = "$0" ]; then
+  detect_pipeline
+fi