chore: stop tracking gitignored tsconfig.tsbuildinfo

This TypeScript incremental-build cache has been committed since
1dcb099 (2021-10-04) but was only added to .gitignore later in
f0a8724 (2022-06-14). Git keeps already-tracked files even once
ignored, so it lingered in the tree — churning on every build and
showing up as spurious diffs. Remove it from version control (the file
stays locally; tsc regenerates it).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rhyslbw