The synlynk exec GitHub CLI guard

synlynk exec now places a short-lived gh shim at the front of the child process's PATH. This closes the remaining Hole B in #1436 without changing the operator's login shell or intercepting ordinary gh commands.

The shim fails closed when neither a role App token nor the explicit SYNLYNK_GH_WRITE_ALLOW_HOST_AUTH opt-in is present. With either token environment variable (GH_TOKEN or GITHUB_TOKEN) set, or with the opt-in, it removes its own directory from PATH, finds the real gh, and execs it.

This keeps role-scoped GitHub writes explicit through synlynk gh --role and prevents a raw child gh from silently using the host nikhilsoman identity.