ci: harden release extraction and version-aware VST build trigger

[thorinf]

Summary

Three small hardening edits on top of PR #42.

Changes

• VST build triggers on VERSION changes — vst/CMakeLists.txt reads VERSION at configure time, so a bump should validate the VST configure/build path
• Flexible changelog header parser — accepts ## 1.0.0, ## v1.0.0, and ## [1.0.0] (Keep-a-Changelog style)
• Random heredoc delimiter — prevents collision if the changelog ever contains a literal CHANGELOG_EOF line

Test plan

• Manually verified the new awk extraction against current CHANGELOG.md — same 1.0.0 body output as before