Correct developer guidance and skip heavy CI for documentation changes (#1090)

Add a minimal AGENTS.md, update documentation and optimize the CI runs to not run actions for which there are no changes present

.github/workflows/ci-changes.yml

@@ -0,0 +1,126 @@
+name: Classify CI changes
+
+on:
+  workflow_call:
+    outputs:
+      build_and_test:
+        description: Run the test and coverage matrix, compatibility tests, and mod build.
+        value: ${{ jobs.changes.outputs.build_and_test }}
+      js_lint:
+        description: Run the JavaScript/TypeScript linter.
+        value: ${{ jobs.changes.outputs.js_lint }}
+      lua_lint:
+        description: Run the Lua linter.
+        value: ${{ jobs.changes.outputs.lua_lint }}
+
+jobs:
+  changes:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    outputs:
+      build_and_test: ${{ steps.syntax.outputs.build_and_test || steps.paths.outputs.code }}
+      js_lint: ${{ steps.lint.outputs.js_lint }}
+      lua_lint: ${{ steps.lint.outputs.lua_lint }}
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+      - uses: dorny/paths-filter@v4
+        id: paths
+        with:
+          token: ''
+          base: ${{ github.event.pull_request.base.sha || github.event.before }}
+          ref: ${{ github.sha }}
+          list-files: json
+          predicate-quantifier: every
+          filters: |
+            # Files requiring validation beyond documentation checks.
+            code:
+              - '**'
+              - '!**/*.md'
+              - '!docs/**/*.{png,jpg,svg,gif}'
+            # Sources and configuration owned by each linter.
+            # Workflow scripts and shared build configuration run both linters.
+            js:
+              - '**/*.{js,cjs,mjs,jsx,ts,cts,mts,tsx}'
+              - '!.github/**'
+              - '!**/{webpack,babel,rollup,vite,jest}.config.*'
+            lua:
+              - '{*.lua,**/*.lua,.emmyrc.json}'
+              - '!.github/**'
+            # Modified JS/TS source eligible for the comment-only shortcut.
+            # TypeScript type changes still count as substantive changes.
+            syntax:
+              - modified: '**/*.{js,cjs,mjs,jsx,ts,cts,mts,tsx}'
+              # Source fixtures and templates always receive full validation.
+              - '!test/file/**'
+              - '!packages/create/templates/**'
+              - '!.github/**'
+              - '!**/{eslint,webpack,babel,rollup,vite,jest}.config.*'
+      - name: Select linters
+        id: lint
+        env:
+          CODE_FILES: ${{ steps.paths.outputs.code_files }}
+          JS_FILES: ${{ steps.paths.outputs.js_files }}
+          LUA_FILES: ${{ steps.paths.outputs.lua_files }}
+        run: |
+          # Unclassified non-documentation files can affect either language.
+          jq -nr --argjson code "$CODE_FILES" --argjson js "$JS_FILES" --argjson lua "$LUA_FILES" '
+            ($code - $js - $lua | length > 0) as $shared |
+            "js_lint=\($shared or ($js | length > 0))",
+            "lua_lint=\($shared or ($lua | length > 0))"
+          ' >> "$GITHUB_OUTPUT"
+      - name: Install Difftastic
+        if: steps.paths.outputs.syntax == 'true'
+        env:
+          DIFFT_VERSION: 0.71.0
+          DIFFT_SHA256: 61aea5394a53c56f144637cf39b3a0c0dafa27769731745e5851ff276e6478da
+        run: |
+          mkdir -p "$RUNNER_TEMP/difftastic"
+          curl -fsSL "https://github.com/Wilfred/difftastic/releases/download/$DIFFT_VERSION/difft-$DIFFT_VERSION-x86_64-unknown-linux-gnu.tar.gz" -o "$RUNNER_TEMP/difftastic.tar.gz"
+          echo "$DIFFT_SHA256  $RUNNER_TEMP/difftastic.tar.gz" | sha256sum -c
+          tar -xzf "$RUNNER_TEMP/difftastic.tar.gz" -C "$RUNNER_TEMP/difftastic"
+          echo "$RUNNER_TEMP/difftastic" >> "$GITHUB_PATH"
+      - name: Check for substantive changes
+        id: syntax
+        if: steps.paths.outputs.code == 'true'
+        env:
+          CODE_FILES: ${{ steps.paths.outputs.code_files }}
+          SYNTAX_FILES: ${{ steps.paths.outputs.syntax_files }}
+          BASE: ${{ github.event.pull_request.base.sha || github.event.before }}
+          HEAD_SHA: ${{ github.sha }}
+        run: |
+          # Skip only when every non-documentation file is an ordinary source edit.
+          echo 'build_and_test=true' >> "$GITHUB_OUTPUT"
+          # A new branch has no previous commit; keep full validation on its first push.
+          if [[ "$BASE" == 0000000000000000000000000000000000000000 ]]; then
+            exit 0
+          fi
+          if ! jq -en --argjson code "$CODE_FILES" --argjson syntax "$SYNTAX_FILES" \
+            '$code - $syntax | length == 0' > /dev/null; then
+            exit 0
+          fi
+          git cat-file -e "$BASE^{commit}"
+          jq -j '.[] | ., "\u0000"' <<< "$SYNTAX_FILES" > "$RUNNER_TEMP/syntax-files"
+          while IFS= read -r -d '' file; do
+            # Mode changes and symlinks are substantive even when text is identical.
+            [[ -z "$(git diff --summary --no-renames "$BASE" "$HEAD_SHA" -- "$file")" ]] || exit 0
+            git ls-tree "$HEAD_SHA" -- "$file" | grep -qE '^100(644|755) blob ' || exit 0
+            before="$RUNNER_TEMP/before.${file##*.}"
+            after="$RUNNER_TEMP/after.${file##*.}"
+            git show "$BASE:$file" > "$before"
+            git show "$HEAD_SHA:$file" > "$after"
+            # Difftastic ignores ALL comments; retain compiler/bundler/coverage directives.
+            if grep -Eq '@ts-|@jsx|@internal|<reference\b|<amd-|sourceMappingURL|sourceURL|webpack|vite|[#@]__|/\*!|(istanbul|c8|v8)[[:space:]]+ignore' "$before" "$after"; then
+              exit 0
+            fi
+            case "$file" in
+              *.js|*.cjs|*.mjs|*.jsx)
+                # JavaScript JSDoc can contain type information.
+                if grep -Fq '/**' "$before" "$after"; then exit 0; fi ;;
+            esac
+            if ! difft --ignore-comments --check-only --exit-code "$before" "$after"; then
+              exit 0
+            fi
+          done < "$RUNNER_TEMP/syntax-files"
+          echo 'build_and_test=false' >> "$GITHUB_OUTPUT"

.github/workflows/lua-lint.yml

@@ -8,7 +8,12 @@ env:
   EMMYLUA_CHECK_VERSION: "0.25.1"
 
 jobs:
+  changes:
+    uses: ./.github/workflows/ci-changes.yml
+
   lint:
+    needs: changes
+    if: ${{ !cancelled() && needs.changes.outputs.lua_lint != 'false' }}
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v7

.github/workflows/node.js.yml

@@ -11,7 +11,12 @@ env:
   NODE_VERSION: 24.x
 
 jobs:
+  changes:
+    uses: ./.github/workflows/ci-changes.yml
+
   test:
+    needs: changes
+    if: ${{ !cancelled() && needs.changes.outputs.build_and_test != 'false' }}
     runs-on: ubuntu-latest
     timeout-minutes: 15
     strategy:
@@ -58,6 +63,8 @@ jobs:
         path: temp/test/logs
 
   version-compat:
+    needs: changes
+    if: ${{ !cancelled() && needs.changes.outputs.build_and_test != 'false' }}
     runs-on: ubuntu-latest
     timeout-minutes: 15
     env:
@@ -90,6 +97,8 @@ jobs:
         --published-version "$PUBLISHED_VERSION"
 
   lint:
+    needs: changes
+    if: ${{ !cancelled() && needs.changes.outputs.js_lint != 'false' }}
     runs-on: ubuntu-latest
     timeout-minutes: 5
     steps:
@@ -101,6 +110,8 @@ jobs:
     - run: pnpm run lint
 
   build:
+    needs: changes
+    if: ${{ !cancelled() && needs.changes.outputs.build_and_test != 'false' }}
     runs-on: ubuntu-latest
     timeout-minutes: 5
     steps: