Function hook_posting_agent_spec 

pub fn hook_posting_agent_spec() -> AgentSpec

A PTY fixture whose script posts a synthetic hook payload to $env:GATE4AGENT_HOOK_URL. Lifecycle hooks are retired (owner ruling 2026-09-25): nothing sets that env var and nothing listens on it anymore, so the post is inert. Kept as a plain PTY fixture for callers that only need its launch shape, not a working hook post.