docs: add release notes for v5.8.18

Author: github-actions[bot]

release_notes/v5.8.18.md

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+## [5.8.18](https://github.com/NHSDigital/eps-common-workflows/compare/v5.8.17...v5.8.18) (2026-03-30)
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+
+### Chore
+
+* [AEA-0000] - workflow to sync copilot instructions (#62) ([f3d19a6](https://github.com/NHSDigital/eps-common-workflows/commit/f3d19a678a725917a5c59cae4d76db621bb7c9c7))
+
+## Info
+[Release workflow run](https://github.com/NHSDigital/eps-common-workflows/actions/runs/23747737357) - Workflow ID: 23747737357
+
+It was initialized by [anthony-nhs](https://github.com/anthony-nhs)
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+