Eight cloud.google.com documentation links across 7 skills return 404

A link check across all skill markdown files found 8 unique `cloud.google.com` documentation URLs that return HTTP 404 (verified twice, following redirects). These appear in the "references"/further-reading sections that agents and users are directed to. In each case a current working page exists — the docs have moved.

| File:line | Dead link (404) | Working replacement (200) |
| :--- | :--- | :--- |
| `skills/cloud/gke-platform-security/SKILL.md:195` | `https://cloud.google.com/kubernetes-engine/docs/how-to/secret-manager` | `https://cloud.google.com/secret-manager/docs/secret-manager-managed-csi-component` |
| `skills/cloud/gke-platform-security/SKILL.md:196` | `https://cloud.google.com/binary-authorization/docs/getting-started-gke` | `https://cloud.google.com/binary-authorization/docs/setting-up` |
| `skills/cloud/gke-platform-security/SKILL.md:198` | `https://cloud.google.com/kubernetes-engine/docs/concepts/sandbox` | `https://cloud.google.com/kubernetes-engine/docs/how-to/sandbox-pods` |
| `skills/cloud/gke-workload-security/SKILL.md:239` | `https://cloud.google.com/kubernetes-engine/docs/how-to/pod-security-standards` | `https://cloud.google.com/kubernetes-engine/docs/how-to/podsecurityadmission` |
| `skills/cloud/gke-workload-security/SKILL.md:240` | `https://cloud.google.com/kubernetes-engine/docs/how-to/secret-manager` | `https://cloud.google.com/secret-manager/docs/secret-manager-managed-csi-component` |
| `skills/cloud/gke-upgrades/SKILL.md:211` | `https://cloud.google.com/kubernetes-engine/docs/how-to/rollout-sequencing` | `https://cloud.google.com/kubernetes-engine/docs/concepts/about-rollout-sequencing` |
| `skills/cloud/google-cloud-global-frontend-configuration/SKILL.md:125` | `https://cloud.google.com/load-balancing/docs/overview` | `https://cloud.google.com/load-balancing/docs` |
| `skills/cloud/alloydb-basics/references/mcp-usage.md:25` | `https://cloud.google.com/alloydb/docs/reference/` | `https://cloud.google.com/alloydb/docs/reference/rest` |
| `skills/cloud/bigtable-basics/SKILL.md:115` | `https://cloud.google.com/bigtable/docs/reference/sql` | `https://cloud.google.com/bigtable/docs/googlesql-overview` |

The suggested replacements are pages verified to return 200 today that cover the same topic — the responsible teams may know a more canonical target.

### Repro

```bash
for u in \
https://cloud.google.com/kubernetes-engine/docs/how-to/secret-manager \
https://cloud.google.com/binary-authorization/docs/getting-started-gke \
https://cloud.google.com/kubernetes-engine/docs/concepts/sandbox \
https://cloud.google.com/kubernetes-engine/docs/how-to/pod-security-standards \
https://cloud.google.com/kubernetes-engine/docs/how-to/rollout-sequencing \
https://cloud.google.com/load-balancing/docs/overview \
https://cloud.google.com/alloydb/docs/reference/ \
https://cloud.google.com/bigtable/docs/reference/sql; do
echo "$(curl -s -o /dev/null -w '%{http_code}' -L "$u") $u"
done
```

Related: #205 (broken *relative* links; this issue covers broken *external* links).

Start with the listed lines in skills/cloud/gke-platform-security/SKILL.md, skills/cloud/gke-workload-security/SKILL.md, skills/cloud/gke-upgrades/SKILL.md, skills/cloud/google-cloud-global-frontend-configuration/SKILL.md, skills/cloud/alloydb-basics/references/mcp-usage.md, and skills/cloud/bigtable-basics/SKILL.md. Run the supplied curl reproduction, update each dead URL to its working replacement, then rerun the checks to confirm the referenced pages no longer return 404.

Written by the indexing model from the issue text.