Function negotiate_hook_support 

pub fn negotiate_hook_support() -> HookSupport

Negotiate the hook tier’s per-OS support level on this host with this build.

Phase 1 (slice 1 of #551) always returns HookSupport::FeatureDisabled because no per-OS injector has landed yet — the feature flag would gate them when they do. Slices 4–6 flip each per-OS branch to Available / Unavailable { reason } honestly.