docs(changelog): May 15 — cleaner, steadier chat

Phone-relevant, default-on fixes only: slash-command chips, no skill
instruction dumps, chat-title race, streaming scroll stability,
permission-prompt clearing, no duplicate history on resume, autocomplete.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>