feat(auto): add agent auto-selection

[skulidropek]

Summary

• replace --claude --auto / --codex --auto with --auto[=claude|codex]
• make bare --auto pick an agent by available auth
• choose Claude when only Claude auth exists, Codex when only Codex auth exists, and randomize when both exist
• reject legacy --claude / --codex flags with a clear parser error

Verification

• pnpm --filter ./packages/app test -- tests/docker-git/parser.test.ts
• pnpm --filter ./packages/lib test -- packages/lib/tests/usecases/agent-auto-select.test.ts packages/lib/tests/usecases/create-project-open-ssh.test.ts
• pnpm check

Notes

• --auto=claude and --auto=codex require matching auth
• bare --auto fails clearly when neither Claude nor Codex auth is available
• old --claude / --codex create/clone flags are intentionally removed