[Security]: upgrade js-yaml to 4.3.1 or remove it from the OpenCode path

## Package

Other — `agent-governance-opencode` / TypeScript SDK dependency path

## Description

The OpenCode package currently pins `js-yaml@4.2.0`. Two GitHub-reviewed high-severity advisories affect that release:

- [GHSA-52cp-r559-cp3m / CVE-2026-59869](https://github.com/advisories/GHSA-52cp-r559-cp3m) affects `js-yaml >=4.0.0,<4.3.0` through crafted YAML merge-key chains.
- [GHSA-5p4m-2wfm-xmqj](https://github.com/advisories/GHSA-5p4m-2wfm-xmqj) affects `js-yaml >=4.0.0,<4.3.1` through quadratic `!!omap` resolution.

Update (2026-09-13): GHSA-2883-xcg3-v3hh / CVE-2026-84375 also affects versions >=4.0.0,<4.3.2. The effective patched floor for the 4.x line is now 4.3.2; 4.3.1 resolves the original two advisories but not this newer one.

Repository-level `overrides` also do not protect downstream consumers of published packages. A clean consumer installation may retain an affected transitive version depending on the published SDK constraints and lockfile.

## How does this impact your work?

A governance integration should avoid inheriting an attacker-controlled CPU-exhaustion path from policy parsing. The OpenCode surface does not itself impose parser resource caps, so downstream consumers need a patched dependency or removal of YAML parsing from that execution path.

## Steps to Reproduce

1. Inspect `agent-governance-opencode/package.json` and its resolved lockfile entry.
2. Run `npm audit` in both the repository package and a clean downstream project installing the published OpenCode package.
3. Observe `js-yaml@4.2.0` or an older transitive version against GHSA-52cp-r559-cp3m.

## Suggested resolution / acceptance criteria

- Upgrade all relevant direct and transitive constraints to `js-yaml >=4.3.2`, or remove YAML parsing from the OpenCode dependency path.
- Regenerate lockfiles and dependency-audit documentation.
- Verify a clean downstream installation, not only the monorepo override.
- Add/retain parser resource-bound tests where AGT consumes untrusted YAML.

## Environment

- Current AGT `main` at commit `81955d4`
- Node 26.7.0
- macOS arm64

## Code of Conduct

- [x] I agree to follow the Microsoft Open Source Code of Conduct

## Coordination status

- Active repository override/lockfile implementation: #3894 (4.3.2 across four CLI packages).
- Earlier OpenCode-only PR #3678 is draft and parked in favor of #3894; do not merge its 4.3.1 pin.
- This issue remains open pending evidence for clean downstream installation and parser resource-bound coverage, or explicitly agreed follow-up tracking.

Please coordinate repository dependency work on #3894 rather than open another competing update. An override-only merge does not by itself complete all acceptance criteria.

Inspect agent-governance-opencode/package.json and its resolved lockfile entry, then review coordination issue #3894 and parked PR #3678 before making changes. Run npm audit in the repository package and a clean downstream project using the published OpenCode package. Done means the patched dependency path or documented removal is verified downstream, lockfiles and audit documentation are updated, and parser resource-bound coverage is present where AGT consumes untrusted YAML.

Written by the indexing model from the issue text.