fix: scope suppress_hook_warning to missing hooks and parse the env override

Parse RTK_SUPPRESS_HOOK_WARNING as a truthy/falsy override that falls
back to config when unset or unrecognised, keep the HookStatus::Ok fast
path free of config loading, and suppress only the missing-hook warning
so the outdated-hook upgrade prompt stays visible.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CNmJZVV9u6pBhcwVhwkPp6