fix: correct broken TOC anchors in plugin-authoring-guide.md

Phase N prefixes were stripped from section headings in a prior commit but the TOC links were left pointing at the old slugs:

#phase-1--core-protocol-required → #core-protocol-required #phase-2--domain-schema → #domain-schema #phase-3--operation-level-merge → #operation-level-merge-ot #phase-4--crdt-semantics → #crdt-semantics