fix(auth): use writable auth paths on ubuntu

[skulidropek]

Summary

• move GitHub and Codex auth mounts off /root and pin GH auth to a single config dir
• look up docker-git SSH keys from the shared projects root before falling back to ~/.ssh
• add regression coverage for auth container paths and key lookup priority

Verification

• pnpm install --frozen-lockfile
• pnpm --filter ./packages/lib test -- packages/lib/tests/usecases/auth-container-paths.test.ts packages/lib/tests/usecases/path-helpers.test.ts packages/lib/tests/usecases/prepare-files.test.ts packages/lib/tests/shell/docker-auth.test.ts
• pnpm --filter ./packages/lib typecheck

Closes #125