Add Dependency Review Action to block vulnerable dependency changes

## Summary

GStack has dependency-bearing manifests (`package.json`, `bun.lock`) but no pull-request dependency review gate. A PR can currently add or bump an npm/Bun dependency without a CI check that blocks newly introduced known vulnerabilities.

GitHub's current dependency review action is designed for this exact PR-time guard: it compares dependency changes in the pull request and can fail the workflow when vulnerable dependencies are introduced.

## Repo-local evidence

- `origin/main` has workflow files under `.github/workflows/`: `actionlint.yml`, `ci-image.yml`, `evals.yml`, `evals-periodic.yml`, `make-pdf-gate.yml`, `pr-title-sync.yml`, `skill-docs.yml`, `version-gate.yml`, `windows-free-tests.yml`, `windows-setup-e2e.yml`.
- `git grep -n -E 'dependency-review|osv-scanner|scorecard|ossf/scorecard|semgrep|codeql-action|github/codeql|attest-build-provenance|attestation|Dependabot' origin/main -- .github package.json bun.lock` returned no matches.
- `origin/main:package.json` includes runtime dependencies (`@huggingface/transformers`, `@ngrok/ngrok`, `diff`, `marked`, `playwright`, `puppeteer-core`, `socks`) and dev dependencies (`@anthropic-ai/claude-agent-sdk`, `@anthropic-ai/sdk`, `xterm`, `xterm-addon-fit`).
- `bun.lock` is tracked on `origin/main`, so dependency changes have a concrete lockfile surface for PR review.
- The repo already has a live dependency-security example: #1588 tracks `diff@7.0.0` / GHSA-73rr-hh4g-fpgx, with #1599 opened as the bump. That kind of issue should be caught at dependency-change time, not only after a separate security review.

## Expected impact

- Blocks PRs that introduce known vulnerable dependencies before they merge.
- Gives contributors immediate feedback in the PR instead of relying on later manual review or ad hoc issue filing.
- Complements #1948: that issue covers secret exposure and mutable action tags; this one covers project dependency vulnerability gating.

## Suggested fix

Add a focused workflow, for example `.github/workflows/dependency-review.yml`:

- Trigger on `pull_request` when `package.json`, `bun.lock`, or the workflow itself changes.
- Use least-privilege `permissions: contents: read`.
- Run `actions/dependency-review-action` with a pinned full commit SHA, not a mutable tag, to stay aligned with #1948.
- Start with `fail-on-severity: moderate` and `fail-on-scopes: runtime, development`, then tune if this is too noisy.

GitHub docs: https://docs.github.com/en/code-security/how-tos/secure-your-supply-chain/manage-your-dependency-security/configuring-the-dependency-review-action

## Duplicate check performed

- Inspected 200 open issues with `gh issue list --repo garrytan/gstack --state open --limit 200 --json number,title,body,url,updatedAt,labels`.
- Inspected 200 open PRs with `gh pr list --repo garrytan/gstack --state open --limit 200 --json number,title,body,url,updatedAt,headRefName,isDraft,labels`.
- Searched open issues/PRs for: `dependency review`, `OSV`, `OpenSSF`, `Scorecard`, `Semgrep`, `workflow permissions`, `least privilege`, `GITHUB_TOKEN permissions`, and `contents: read`.
- No open issue or PR covers adding a dependency review gate. Adjacent items found:
- #1948 covers provider API key exposure and mutable action tags.
- #1588 / #1599 cover one specific vulnerable dependency bump.
- #663 covers PR-comment workflow permissions for `evals.yml`, not dependency vulnerability gating.

## Commands/files inspected

- `gh issue list --repo garrytan/gstack --state open --limit 200 --json number,title,body,url,updatedAt,labels`
- `gh pr list --repo garrytan/gstack --state open --limit 200 --json number,title,body,url,updatedAt,headRefName,isDraft,labels`
- `gh api --method GET search/issues -f q=''`
- `git fetch origin main`
- `git ls-tree -r --name-only origin/main .github`
- `git grep -n -E 'dependency-review|osv-scanner|scorecard|ossf/scorecard|semgrep|codeql-action|github/codeql|attest-build-provenance|attestation|Dependabot' origin/main -- .github package.json bun.lock`
- `git show origin/main:package.json`
- `git ls-tree -r --name-only origin/main | egrep '(^|/)(bun.lockb|bun.lock|package-lock.json|pnpm-lock.yaml|yarn.lock)$'`

Start by reading the existing workflows under .github/workflows/ and the dependency manifests package.json and bun.lock. Add the proposed .github/workflows/dependency-review.yml with the stated pull-request paths, least-privilege permissions, pinned action revision, and severity and scope settings. Done means dependency-changing pull requests run the check and newly introduced vulnerable dependencies can block the workflow.

Written by the indexing model from the issue text.