feat(release): sharpen next release announcements

[fcoury-oai]

Why

The next Red release has several substantial Agent and Vim editing changes, but the existing campaign renders them as a flat feature list. The public website also still describes an older Agent workflow, so pointing the launch posts there or reusing its screenshot would mislead readers.

What changed

• Curate nine release stories around source-linked Agent walkthroughs, scoped inline help, multi-cursor editing, Agent LSP tools, model selection, large-workspace speed, external-file protection, Vim shell filters, and guided onboarding.
• Lead the GitHub release introduction with three concrete workflows. Give Discord a focused opening, grouped secondary highlights, and a question for the community.
• Use reviewed, character-bounded X and Bluesky copy and link those posts to GitHub Releases until the website is current. Discord omits the stale image unless a verified discord_image URL is provided.

This PR keeps the campaign at version = "next". It does not publish anything: the separate release-preparation workflow resolves the chosen version, and the release tag/publication flow remains a later action.

How to Test

1. Run python3 scripts/release_campaign.py validate and python3 scripts/release_campaign.py render --all --output-dir /private/tmp/red-campaign-preview. Check the GitHub opening, Discord story order, and distinct X/Bluesky previews. The social renderers reject posts over their platform limits.
2. Run PYTHONDONTWRITEBYTECODE=1 PYTHONPATH=. python3 -m unittest discover -s tests -p 'test_*release*.py'. The 39 focused tests cover the reviewed campaign, resolved-version social lengths, Discord media omission/override, and fallback behavior.
3. Before release preparation, review the nine stories against the final tag, capture a current Agent walkthrough poster frame for discord_image, and confirm the destination link reflects the published release.