fix: make beam installer and daemon launch portable

[ejgallego]

Summary

• make plugin library naming platform-aware instead of hardcoding .so
• replace install symlinks portably instead of relying on GNU mv -T
• launch beam-daemon via IO.Process.spawn with direct pid capture, removing shell backgrounding via & echo $!
• drop the rg dependency from tests/test-install.sh

Why

The macOS investigation surfaced a chain of portability bugs in the install/startup path:

• plugin path assumptions were Linux-only
• symlink replacement relied on GNU mv -T
• daemon startup relied on sh -c '... & echo $!', which hung before pid capture / registry write on macOS

This PR keeps the fix surface targeted to those portability issues.

Validation

• lake build beam-cli
• bash scripts/lint-shell.sh
• targeted stale-import repro with BEAM_REQUEST_ID=install-stale-sync returned syncBarrierIncomplete locally after the spawn-based launch change