docs(release): move announcements to the web repo's announce skill

[datlechin]

Why

The release skill mixed cutting a release with writing its blog post, newsletter and X posts. The post lives in TableProApp/web, and the moved guidance had gone stale there (figure tags, BlogTest, docs paths in lint-draft.py).

Change

• SKILL.md drops the shape decision, the announce stages and the announce triggers; stages are now Release and Plugins, with a pointer to the web repo's announce skill
• Deletes references/ and scripts/lint-draft.py, now in feat(skills): add the announce skill for release posts, newsletters and X web#116
• check-doc-symbols.sh drops the marketing site exemption that only blog-post.md used
• banned-words.txt stops naming lint-draft.py as a reader

Verified

verify.sh agent-docs and check-banned-words.sh --staged pass.

Not in this PR

• Merge after feat(skills): add the announce skill for release posts, newsletters and X web#116.