chore: stop tracking .github/announce/ — keep launch drafts local

These were never meant for public consumption: HN/Reddit body templates,
endorser cold-email scripts, posting-time recommendations, reply tips.
Leaving them visible undercuts the protocol's serious-engineering framing.

Files are kept on disk locally for the author's reference; just no longer
in the public repo. Historical commits still contain them, which is fine
(nobody mines old commits).

Author: WaylandYang