Function install_bun_shell 

pub unsafe fn install_bun_shell(
    cx: &mut JSContext,
    bun_obj: Handle<'_, *mut JSObject>,
)

Install Bun.Shell and Bun.$ on the global Bun object. Replaces the Phase 1 stub with a full implementation using bun_shell_parser + bun_spawn (SPEC: REQ-BAO-API-018 reuse mapping).

@trace REQ-BAO-API-018 [api:Bun.Shell/$ installation]