feat: automate release notes and refresh readme hero

[cafitac]

Summary

• render GitHub Release bodies from a shared script so the auto-publish path and the tag/manual fallback use the same planner/executor framing
• replace the README text-only opening with a dedicated SVG hero asset and tighter landing-page copy
• document the new hero/release-note framing in the positioning and changelog docs

Verification

• python3 -m py_compile scripts/render_release_notes.py
• python3 scripts/render_release_notes.py --tag v0.3.54 --version 0.3.54 --repo cafitac/hermit-agent --reason 'manual verification'
• git diff --check
• magick docs/assets/hermit-readme-hero.svg docs/assets/hermit-readme-hero.png
• YAML parse of .github/workflows/publish-npm.yml and publish-github-release.yml