When Bun is installed on the system but QMD was installed via npm,
$BUN_INSTALL is always set (typically to ~/.bun), causing the launcher
to incorrectly run QMD under Bun. This leads to ABI mismatches with
native modules (better-sqlite3, sqlite-vec) that were compiled for Node,
breaking vector operations with "no such module: vec0".
Only check for bun.lock/bun.lockb files, which reliably indicate that
QMD was actually installed with Bun.
Fixes#361
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>