[codex] Add CI-gated Dependabot automation

.github/dependabot.yml

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+version: 2
+updates:
+- package-ecosystem: "pip"
+  directory: "/"
+  schedule:
+    interval: "weekly"
+  open-pull-requests-limit: 5
+  ignore:
+  - dependency-name: "*"
+    update-types:
+    - "version-update:semver-major"
+- package-ecosystem: "github-actions"
+  directory: "/"
+  schedule:
+    interval: "weekly"
+  open-pull-requests-limit: 5
+  ignore:
+  - dependency-name: "*"
+    update-types:
+    - "version-update:semver-major"

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@v6
+      - uses: actions/setup-python@v6
+        with:
+          python-version: '3.11'
+      - name: Install package
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -e '.[test]'
+      - name: Run tests
+        run: python -m pytest -q

.github/workflows/dependabot_auto_merge.yml

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+name: Auto Merge Dependabot PR
+
+"on":
+  workflow_run:
+    workflows: ["CI"]
+    types: [completed]
+
+jobs:
+  auto-merge:
+    if: github.event.workflow_run.conclusion == 'success' && startsWith(github.event.workflow_run.head_branch, 'dependabot/')
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    permissions:
+      contents: write
+      pull-requests: write
+
+    steps:
+      - name: Resolve Dependabot PR
+        id: pr
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          set -euo pipefail
+          BRANCH_NAME="${{ github.event.workflow_run.head_branch }}"
+          PR_NUMBER=$(gh pr list --repo "${GITHUB_REPOSITORY}" --state open --head "${BRANCH_NAME}" --json number --jq '.[0].number // empty')
+          if [ -z "${PR_NUMBER}" ]; then
+            echo "No open Dependabot PR found for ${BRANCH_NAME}." >> "$GITHUB_STEP_SUMMARY"
+            exit 0
+          fi
+          echo "pr_number=${PR_NUMBER}" >> "$GITHUB_OUTPUT"
+
+      - name: Evaluate merge eligibility
+        id: merge_guard
+        if: steps.pr.outputs.pr_number != ''
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          set -euo pipefail
+          gh pr view "${{ steps.pr.outputs.pr_number }}" --repo "${GITHUB_REPOSITORY}" --json number,isDraft,author,url,body,labels > pr.json
+          python3 - <<'PY'
+          import json
+          import os
+          from pathlib import Path
+
+          pr = json.loads(Path("pr.json").read_text(encoding="utf-8"))
+          author = (pr.get("author") or {}).get("login")
+          labels = {item.get("name", "") for item in pr.get("labels", [])}
+          body = pr.get("body") or ""
+          is_major = "update-type: version-update:semver-major" in body
+          dependabot_authors = {"dependabot[bot]", "app/dependabot"}
+          is_dependabot = author in dependabot_authors and "dependencies" in labels
+          should_merge = is_dependabot and not pr.get("isDraft") and not is_major
+          if should_merge:
+              reason = "ready"
+          elif is_major:
+              reason = "major_update"
+          else:
+              reason = "not_eligible_dependabot_pr"
+
+          summary_lines = [
+              "## Auto-Merge Gate",
+              f"- PR: {pr['url']}",
+              f"- Author: `{author or '<unknown>'}`",
+              f"- Draft: `{'yes' if pr.get('isDraft') else 'no'}`",
+              f"- Dependabot label: `{'yes' if 'dependencies' in labels else 'no'}`",
+              f"- Major update: `{'yes' if is_major else 'no'}`",
+              f"- Final merge decision: `{'merge' if should_merge else 'skip'}`",
+              f"- Reason: `{reason}`",
+          ]
+          Path("pr-summary.md").write_text("\n".join(summary_lines).strip() + "\n", encoding="utf-8")
+          with open(os.environ["GITHUB_OUTPUT"], "a", encoding="utf-8") as output:
+              print(f"should_merge={'true' if should_merge else 'false'}", file=output)
+              print(f"reason={reason}", file=output)
+          PY
+
+      - name: Append merge summary
+        if: steps.pr.outputs.pr_number != ''
+        run: cat pr-summary.md >> "$GITHUB_STEP_SUMMARY"
+
+      - name: Merge Dependabot PR
+        if: steps.merge_guard.outputs.should_merge == 'true'
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: gh pr merge "${{ steps.pr.outputs.pr_number }}" --repo "${GITHUB_REPOSITORY}" --rebase --delete-branch