chore: gitignore bun.lock

Bun auto-generates `bun.lock` when its scripts run (`bun run build`,
`bun run lint`). The project's web package manager is npm
(`package-lock.json`), so the bun lockfile is incidental — not the
source of truth. Ignore it to keep the working tree clean for anyone
who happens to invoke bun.

Signed-off-by: Chris Pardy <chris.pardy@gmail.com>