v0.4.0

• cost now attributes prompt sections: the mounted plugin observes the
  system-prompt/assemble waterfall (purely — the assembly passes through
  unmodified) and records per-section token estimates alongside tool schemas.
  Output separates section vs schema totals; empty state explains that one
  agent message is needed before sections appear.
• xray_composition summary and cost view carry the blended totals.
• README: terminal demo image plus real cost output.