fix(ci): switch to permissionless reusable workflow (#335)

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## Summary

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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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## 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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----
## Summary by Gitar

- **CI / Tooling:**
- Updated `secret-scanner.yml` workflow to use a permissionless reusable
workflow reference


<sub>This will update automatically on new commits.</sub>

Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>

Author: hyperpolymath

.github/workflows/secret-scanner.yml

@@ -15,8 +15,6 @@ permissions:
 
 jobs:
   scan:
-    # The reusable's gitleaks job requests pull-requests: write (PR summary
-    # comment) and actions: read (workflow-run metadata) at job level; the
     # caller must grant at least that or the run startup-fails.
     permissions:
       contents: read