chore: remove CHANGELOG.md again and gitignore it (#3803)

#2922 removed CHANGELOG.md in favor of generated release notes, and #3593 recreated it. Since
then it has made parallel PRs conflict (#3732 three times, #3738), and it adds nothing that the
release workflow does not already produce with `gh release --generate-notes`.

Delete it and add it to .gitignore, so `git add CHANGELOG.md` or `git add .` refuses it.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: thymikee

.gitignore

@@ -56,3 +56,6 @@ packages/*/dist-types/
 
 # Reproducible AX bridge captures are published on the evidence branch.
 docs/evidence/ios-simulator-ax-bridge-*.json.gz
+
+# Release notes are generated from PR titles; a shared changelog conflicts on every PR.
+CHANGELOG.md