Add GitHub Actions reusable workflow for @ghstack merge

.github/merge_rules.yaml

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+# Merge rules for ghstack land
+#
+# Rules are matched in order - first matching rule wins.
+# Each rule specifies:
+#   - name: A descriptive name for the rule
+#   - patterns: File glob patterns to match (fnmatch syntax)
+#   - approved_by: Required approvers (usernames or org/team refs)
+#                  Use "any" to accept any approved review
+#   - mandatory_checks_name: CI checks that must pass
+#   - ignore_flaky_failures: (optional) If true, ignore check failures
+
+- name: core-library
+  patterns:
+    - "src/ghstack/**"
+  approved_by:
+    - ezyang
+    - pytorch/ghstack-maintainers  # Team reference example
+  mandatory_checks_name:
+    - "Test"
+    - "Lint"
+
+- name: documentation
+  patterns:
+    - "docs/**"
+    - "*.md"
+  approved_by:
+    - any  # Any approved review is sufficient
+  mandatory_checks_name:
+    - "Lint"
+
+- name: ci-config
+  patterns:
+    - ".github/**"
+  approved_by:
+    - ezyang
+  mandatory_checks_name:
+    - "Test"
+    - "Lint"
+
+- name: default
+  patterns:
+    - "*"
+    - "**/*"
+  approved_by:
+    - ezyang
+  mandatory_checks_name:
+    - "Test"
+    - "Lint"

.github/workflows/ghstack-merge-on-comment.yml

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+# Example workflow that calls the reusable ghstack-merge workflow
+# when someone comments "@ghstack merge" on a PR.
+#
+# To use in your repository:
+# 1. Copy this file to .github/workflows/
+# 2. Optionally create .github/merge_rules.yaml for merge rules
+# 3. Comment "@ghstack merge" on any ghstack PR to trigger landing
+
+name: ghstack merge on comment
+
+on:
+  issue_comment:
+    types: [created]
+
+jobs:
+  check-comment:
+    # Only run on PR comments containing '@ghstack merge'
+    if: |
+      github.event.issue.pull_request &&
+      contains(github.event.comment.body, '@ghstack merge')
+    runs-on: ubuntu-latest
+    outputs:
+      should_merge: ${{ steps.check.outputs.should_merge }}
+    steps:
+      - name: Check if comment triggers merge
+        id: check
+        run: echo "should_merge=true" >> $GITHUB_OUTPUT
+
+  merge:
+    needs: check-comment
+    if: needs.check-comment.outputs.should_merge == 'true'
+    uses: ezyang/ghstack/.github/workflows/ghstack-merge.yml@master
+    with:
+      pull_request_number: ${{ github.event.issue.number }}
+      validate_rules: true
+      comment_on_failure: true
+    secrets:
+      github_token: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/ghstack-merge.yml

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+name: ghstack merge
+
+on:
+  workflow_call:
+    inputs:
+      pull_request_number:
+        description: 'The pull request number to land'
+        required: true
+        type: number
+      python_version:
+        description: 'Python version to use'
+        required: false
+        type: string
+        default: '3.11'
+      ghstack_version:
+        description: 'ghstack version to install (e.g., "ghstack", "ghstack==0.14.0", "git+https://github.com/ezyang/ghstack.git")'
+        required: false
+        type: string
+        default: 'ghstack'
+      validate_rules:
+        description: 'Whether to validate against merge rules'
+        required: false
+        type: boolean
+        default: true
+      dry_run:
+        description: 'Validate only, do not actually merge'
+        required: false
+        type: boolean
+        default: false
+      comment_on_failure:
+        description: 'Post validation errors as PR comment'
+        required: false
+        type: boolean
+        default: true
+      runs_on:
+        description: 'Runner to use'
+        required: false
+        type: string
+        default: 'ubuntu-latest'
+    secrets:
+      github_token:
+        description: 'GitHub token with contents:write and pull-requests:write permissions'
+        required: true
+
+jobs:
+  merge:
+    runs-on: ${{ inputs.runs_on }}
+    permissions:
+      contents: write
+      pull-requests: write
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          token: ${{ secrets.github_token }}
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ inputs.python_version }}
+
+      - name: Install ghstack
+        run: pip install '${{ inputs.ghstack_version }}'
+
+      - name: Configure git
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+
+      - name: Build ghstack land command
+        id: build-command
+        run: |
+          CMD="ghstack land"
+          if [ "${{ inputs.validate_rules }}" = "true" ]; then
+            CMD="$CMD --validate-rules"
+          fi
+          if [ "${{ inputs.dry_run }}" = "true" ]; then
+            CMD="$CMD --dry-run"
+          fi
+          if [ "${{ inputs.comment_on_failure }}" = "true" ]; then
+            CMD="$CMD --comment-on-failure"
+          fi
+          CMD="$CMD ${{ inputs.pull_request_number }}"
+          echo "command=$CMD" >> $GITHUB_OUTPUT
+
+      - name: Run ghstack land
+        env:
+          GITHUB_TOKEN: ${{ secrets.github_token }}
+        run: ${{ steps.build-command.outputs.command }}