fix: only update existing tsconfig.json, don't create new ones
Update existing tsconfig.json to add extends: ./tsconfig.deno.json,
but don't create a new tsconfig.json. Creating one interfered with
Deno's own type checker which would pick up the "types": ["deno"]
setting from the extended config.
Also remove dead code warning by keeping update_user_tsconfig in use.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
