Update project to use Bun as the package manager and add bunfig.toml for dependency management

Commit notes

- Replaced pnpm commands with bun commands in AGENTS.md and package.json. - Added bunfig.toml to specify trusted dependencies and ensure reproducibility. - Updated bun.lock file to reflect new dependencies and removed pnpm-lock.yaml and pnpm-workspace.yaml for cleanup. - Updated backend package.json scripts to utilize bunx for pretty logging.