Function compute_body_hash 

pub fn compute_body_hash(body: &str) -> String

Compute the content hash of an agent body (its system_prompt).

32-byte blake3, hex-encoded. This is the same form that populates AgentProfile.version_hash, and the same form recomputed by the patch_applier.lua post-hook when it detects a /agents/N/profile/system_prompt replacement — the host.content_hash primitive is also blake3 — so the Phase 1 hash-consistency guarantee holds.