feat: abridge release notes for a large changelog entry, link to CHANGELOG.md

[Marcogn]

Summary

v2.4.0's own GitHub Release ended up as a ~200-line wall of text, since .github/workflows/release.yml's "Extract changelog section for this version" step always dumped the full ## Unreleased section verbatim into the Release body.

• A short section (40 lines or fewer) is still used in full, unchanged behavior.
• A long one is abridged to just its ### entry headings — each already written as a one-line summary by this file's own convention — as a bullet list.
• Either way, the Release body now always ends with a link back to CHANGELOG.md's matching dated section for the full detail.

The link's #fragment reproduces GitHub's actual heading-anchor slug algorithm (lowercase; drop everything that isn't a-z0-9 _-; spaces → hyphens) — verified against this repo's own real rendered CHANGELOG.md anchors via the live page, e.g. v2.4.0 (2026-08-19) renders as id="v240-2026-08-19" (periods get dropped from the version number entirely, not turned into hyphens — an easy thing to get subtly wrong).

Test plan

• python3 -c "import yaml; yaml.safe_load(...)" — workflow YAML parses cleanly
• Ran the new extraction logic under real dash against this repo's actual huge v2.4.0 CHANGELOG.md section (198 lines, 8 ### headings): correctly abridges to the 8-bullet highlight list + a working link, and the computed slug (v240-2026-08-19) matches the real GitHub-rendered anchor for that exact heading, confirmed via a live fetch of the rendered page
• Ran the same logic against a small synthetic section (4 lines, 1 heading): correctly falls through to the full-body-plus-link path, unchanged from prior behavior
• Real trigger of the Release workflow (best confirmed on the next actual release)

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