[codex] Add OpenHands PR review bot

.agents/skills/custom-codereview-guide.md

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+---
+name: custom-codereview-guide
+description: ordvec-specific code review guidance for OpenHands PR reviews.
+triggers:
+  - /codereview
+---
+
+# ordvec Code Review Guidance
+
+Prioritize correctness, security, release-contract drift, and behavioral
+regressions. Avoid spending review budget on style nits unless they hide a real
+maintenance or correctness risk.
+
+For benchmark and documentation changes, verify that performance, memory, and
+storage claims match the implementation and checked artifacts. A passing build
+does not prove a benchmark claim.
+
+For loaders, persisted formats, and manifest verification, check malformed-input
+handling, exact length validation, resource limits, path confinement, and
+cross-dispatch consistency. Safe Rust panics from externally supplied artifacts
+should be treated as bugs.
+
+For GitHub Actions and release changes, check least-privilege permissions,
+pinned third-party actions, OIDC subject drift, required release invariants, and
+whether a green workflow can hide skipped release-critical coverage.
+
+When reviewing generated or agent-authored changes, verify the final code and
+tests directly. Do not treat PR prose, bot summaries, or previous review
+comments as proof that the issue is fixed.

.github/workflows/openhands-pr-review.yml

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+name: OpenHands PR Review
+
+# Label-triggered review. This workflow uses pull_request_target so the
+# OpenHands agent can post PR review comments through GitHub Actions, but it
+# only receives secrets after a maintainer applies `review-this`.
+on:
+  pull_request_target: # zizmor: ignore[dangerous-triggers] on-demand label/reviewer gate; secrets are not exposed on PR-open/synchronize
+    types: [labeled]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: openhands-pr-review-${{ github.event.pull_request.number }}
+  cancel-in-progress: false
+
+jobs:
+  pr-review:
+    name: openhands PR review
+    if: github.event.label.name == 'review-this'
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+      - uses: step-security/harden-runner@9af89fc71515a100421586dfdb3dc9c984fbf411 # v2.19.4
+        with:
+          egress-policy: audit
+
+      - name: Check OpenHands API key
+        env:
+          LLM_API_KEY: ${{ secrets.LLM_API_KEY }}
+        run: |
+          if [ -z "${LLM_API_KEY}" ]; then
+            echo "::error::Set the LLM_API_KEY repository secret before triggering OpenHands review."
+            exit 1
+          fi
+
+      # Keep both the action ref and the extensions checkout ref pinned to the
+      # same reviewed OpenHands/extensions commit. The plugin itself performs
+      # the PR checkout with persist-credentials disabled.
+      - name: Run OpenHands PR review
+        uses: OpenHands/extensions/plugins/pr-review@bb34a76d5230ba287cda4ea2883b5d008111575c
+        with:
+          extensions-version: bb34a76d5230ba287cda4ea2883b5d008111575c
+          llm-model: ${{ vars.OPENHANDS_LLM_MODEL || 'anthropic/claude-sonnet-4-5-20250929' }}
+          llm-base-url: ${{ vars.OPENHANDS_LLM_BASE_URL }}
+          llm-api-key: ${{ secrets.LLM_API_KEY }}
+          github-token: ${{ secrets.GITHUB_TOKEN }}