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

scripts/test-detect-pipeline.sh

@@ -0,0 +1,110 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "$SCRIPT_DIR/detect-pipeline.sh"
+
+PASS=0
+FAIL=0
+
+assert_eq() {
+  local test_name="$1" expected="$2" actual="$3"
+  if [ "$expected" = "$actual" ]; then
+    echo "PASS: $test_name"
+    PASS=$((PASS + 1))
+  else
+    echo "FAIL: $test_name (expected=$expected, actual=$actual)"
+    FAIL=$((FAIL + 1))
+  fi
+}
+
+run_detect() {
+  echo "$1" | detect_pipeline
+}
+
+# --- Pipeline B cases (infra-only) ---
+
+assert_eq "single .github root file" "pipeline-b" \
+  "$(run_detect ".github/dependabot.yml")"
+
+assert_eq ".github/workflows subdirectory" "pipeline-b" \
+  "$(run_detect ".github/workflows/ci.yaml")"
+
+assert_eq "deeply nested .github path" "pipeline-b" \
+  "$(run_detect ".github/workflows/reusable/build.yaml")"
+
+assert_eq "markdown file at root" "pipeline-b" \
+  "$(run_detect "README.md")"
+
+assert_eq "markdown in subdirectory" "pipeline-b" \
+  "$(run_detect "docs/CONTRIBUTING.md")"
+
+assert_eq "eslintrc config" "pipeline-b" \
+  "$(run_detect ".eslintrc.json")"
+
+assert_eq "prettierrc config" "pipeline-b" \
+  "$(run_detect ".prettierrc.yaml")"
+
+assert_eq "renovate config" "pipeline-b" \
+  "$(run_detect "renovate.json")"
+
+assert_eq "renovate config5" "pipeline-b" \
+  "$(run_detect "renovate.json5")"
+
+assert_eq "gitignore" "pipeline-b" \
+  "$(run_detect ".gitignore")"
+
+assert_eq "editorconfig" "pipeline-b" \
+  "$(run_detect ".editorconfig")"
+
+assert_eq "LICENSE" "pipeline-b" \
+  "$(run_detect "LICENSE")"
+
+assert_eq "mixed infra files" "pipeline-b" \
+  "$(run_detect ".github/workflows/ci.yaml
+README.md
+.eslintrc.json
+LICENSE")"
+
+assert_eq "workflow + markdown combo" "pipeline-b" \
+  "$(run_detect ".github/workflows/detect-pr-pipeline.yaml
+.github/workflows/README.md")"
+
+# --- Pipeline A cases (has non-infra files) ---
+
+assert_eq "plugin source file" "pipeline-a" \
+  "$(run_detect "headlamp-polaris-plugin/src/index.tsx")"
+
+assert_eq "plugin package.json" "pipeline-a" \
+  "$(run_detect "headlamp-polaris-plugin/package.json")"
+
+assert_eq "root source file" "pipeline-a" \
+  "$(run_detect "src/main.ts")"
+
+assert_eq "mixed infra + code" "pipeline-a" \
+  "$(run_detect ".github/workflows/ci.yaml
+headlamp-polaris-plugin/src/index.tsx
+README.md")"
+
+assert_eq "single non-infra file" "pipeline-a" \
+  "$(run_detect "server.js")"
+
+# --- Edge cases ---
+
+assert_eq "empty input" "pipeline-b" \
+  "$(run_detect "")"
+
+assert_eq "root dot file (not in infra list)" "pipeline-a" \
+  "$(run_detect ".env")"
+
+assert_eq ".github-like but not .github dir" "pipeline-a" \
+  "$(run_detect ".github-backup/config.yaml")"
+
+# --- Summary ---
+
+echo ""
+echo "Results: $PASS passed, $FAIL failed"
+
+if [ "$FAIL" -gt 0 ]; then
+  exit 1
+fi