fix(rfc): pass 3 — rewrite Sections 18, 19, 20 to match implementation
Section 18: Removed PERFORM references, ActionDef→Action, TaskDef→Task,
OnStartup returns Result, removed aspirational patterns (Parallel,
Conditional, Scheduled), kept what's implemented.
Section 19: Removed old struct definition, Stream returns Result not
(Stream, error), RemoteAction uses c.RemoteAction() not c.Action(),
removed stale subsystem map, added Web3 snider.lthn example.
Section 20: Removed migration history (20.1 The Problem, 20.6-20.8),
kept the API contract. Added 20.6 "What Embeds Registry" reference.
4193 → 1519 lines (64% reduction total).
Co-Authored-By: Virgil <virgil@lethean.io>