ci(.github/workflows): update checkout to v7 (#26909)

Update GitHub Actions workflows to use `actions/checkout` v7.0.0 pinned
to `9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0`, following the GitHub
Actions checkout hardening changes announced in:

-
https://github.blog/changelog/2026-06-18-safer-pull_request_target-defaults-for-github-actions-checkout/
-
https://github.blog/changelog/2026-06-18-control-who-and-what-triggers-github-actions-workflows/

Audited the existing `pull_request_target` workflows and did not add any
`allow-unsafe-pr-checkout` opt-outs, since these workflows do not
intentionally check out fork PR head code.

Generated by Coder Agents.

<details>
<summary>Plan notes</summary>

- Update all `.github/workflows` `actions/checkout` references to v7.0.0
using the pinned SHA.
- Preserve SHA pinning, including the newly added MCP registry workflow.
- Validate that old checkout pins are removed and no unsafe checkout
opt-outs are introduced.

</details>

.github/workflows/deploy.yaml

@@ -30,7 +30,7 @@ jobs:
           egress-policy: audit
 
       - name: Checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
         with:
           fetch-depth: 0
           persist-credentials: false
@@ -59,7 +59,7 @@ jobs:
           egress-policy: audit
 
       - name: Checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
         with:
           fetch-depth: 0
           persist-credentials: false