Add automated code review workflow (#96359)
## Summary
Adds one protected `pull_request_target` workflow for advisory automated
review of non-draft, member-authored PRs.
The existing Vercel reviewer runs Codex, Claude, and synthesis with
read-only GitHub access. This base-owned workflow treats that result as
untrusted, validates it, and owns the only GitHub mutation.
## Write boundary
- no checkout, PR code execution, dependency installation, Actions
secrets, or shell interpolation
- globally empty permissions; the job has only OIDC and pull-request
read
- exact repository, event, author association, PR state, base SHA, and
head SHA validation
- final read-only PR revalidation immediately before publishing
- HTML comments stripped, mentions neutralized, token patterns redacted,
and output capped at 48 KB
- a write token is minted only after validation
- exactly one top-level comment `POST` or `PATCH`
- existing comments are eligible only when attributed to one of gh-sts's
immutable `general` GitHub App IDs and begin with the fixed marker
- both actions are pinned to immutable commit SHAs
The workflow intentionally runs once on `opened` or `ready_for_review`;
it does not run on `synchronize`.
## Validation
- `actionlint`
- Prettier
- syntax validation of all three embedded `github-script` programs
- upstream commit verification for both pinned actions
- independent Codex and Claude autoreview
The workflow remains inert until the matching gh-sts policy is approved.
