fix(ci): write each paragraph of the nightly release notes on one line

[devin-ai-integration]

What and why

The nightly snapshot's release notes were written with the source wrapped at ~90 columns. GitHub renders a release body with hard line breaks, so the published notes broke mid-sentence after every source line. The "Write the release notes" step now emits each prose paragraph as a single line; the code block and the changelog summary are unchanged. $count commit(s) becomes 1 commit / N commits.

How it was verified

Ran the step's script locally with the workflow's environment variables set and confirmed each paragraph is one line and the rest of the body is byte-identical in structure.

Checklist

• make test and make lint pass locally (workflow-only change; python3 scripts/changelog.py check passes)
• Tests added or updated for the change (none cover the workflow)
• Documentation extended where it already covers the surface (see CONTRIBUTING.md)
• Changelog entry added as changes/unreleased/<slug>.<section>.md, not as an edit to CHANGELOG.md
• baselines regenerated and make docs-counts run if a gate count moved (compliance rows need nothing: the census is counted at docs build)
• No internal work-item labels (waves, slices, F4, K5) in the body, docs, or changelog

[devin-ai-integration]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.