Remove checked-in shell completion files

[jimpudar]

Summary

• Delete the generated bash and zsh completion artifacts from the repo.
• Remove the bun run completions refresh script and update the README to point at on-demand generation from the installed rootcell binary.
• Switch repo-root detection and tests to rely on source layout instead of the removed completions/ directory.

Testing

• bun run test -- src/rootcell/rootcell.test.ts
• bun run test
• bun run typecheck
• bun run lint