chore(dependabot): cap open pull requests per update block (#139)

Adds `open-pull-requests-limit` to Dependabot update blocks that had no
cap, following the estate per-ecosystem cap doctrine (task #37). No
other line in the file is touched.


Claude-Session: https://claude.ai/code/session_011eQ7hibx92N7fBDtwgReWk

<!--
SPDX-License-Identifier: CC-BY-SA-4.0
Copyright (c) Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
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## Summary

<!-- What does this PR do, and why? -->

Closes #

## Type of change

- [ ] 🐛 Bug fix (non-breaking change that fixes an issue)
- [ ] ✨ New feature (non-breaking change that adds functionality)
- [ ] 💥 Breaking change (would change existing behaviour)
- [ ] 🕳️ Soundness fix (fixes a checker/proof false-negative)
- [ ] 📖 Documentation
- [ ] 🧹 Refactor / tech debt (behaviour-preserving)
- [ ] ⚡ Performance
- [ ] 🔧 Build / CI / tooling

## How has this been verified?

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report?
     Don't cite a status doc — cite the command and its output. -->

## Checklist

- [ ] My commits are **signed** (`git commit -S`).
- [ ] I ran the project's own checks/tests locally and they pass.
- [ ] New files carry the correct `SPDX-License-Identifier` (code/config
`MPL-2.0`,
      prose `CC-BY-SA-4.0`); I did not relicense existing files.
- [ ] Docs are updated, and no public claim now overstates what the code
does.
- [ ] I have not introduced a soundness hole (or I have flagged where I
might have).

## Notes for reviewers

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Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: hyperpolymath

.github/dependabot.yml

@@ -30,11 +30,13 @@ updates:
       actions:
         patterns:
           - "*"
+    open-pull-requests-limit: 2
   - package-ecosystem: "guix"
     directory: "/"
     schedule:
       interval: "weekly"
   - package-ecosystem: "npm"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "weekly"
+    open-pull-requests-limit: 3