Follow up remaining portability cleanup in wrapper and test infra

[ejgallego]

Context

PR #14 fixes the highest-impact macOS portability problems in the installer and daemon startup path, including replacing shell backgrounding in daemon launch.

This follow-up tracks the smaller remaining portability cleanup items that surfaced during the same investigation.

Remaining cleanup

• Replace the remaining helper sh -c calls in Beam/Cli.lean where practical.
  Current examples include PATH lookup and pid liveness / kill probes.
  These are lower risk than the old daemon launch path, but they are still shell-dependent wrapper behavior.

• Make tests/test-beam-wrapper.sh portable with respect to sed -i.
  BSD sed and GNU sed differ here, and this script still assumes GNU semantics.

• Review whether other test helpers should avoid optional external tool assumptions unless CI installs them explicitly.
  The rg issue in tests/test-install.sh was fixed, but the broader pattern is worth checking.

• Audit whether any other installer / wrapper shell helpers duplicate platform logic that could be centralized.
  PR fix: make beam installer and daemon launch portable #14 already deduplicates native-library naming into scripts/shared-lib.sh, but the same principle may apply elsewhere.

Goal

Keep the public lean-beam surface stable while reducing the number of platform-specific surprises in wrapper and test infrastructure.

[ejgallego]

Closing this because the remaining portability cleanup is being folded directly into PR #14 instead of tracked separately.