content(agentcore): revise to June 2026 — fold in harness/Managed KB/Web Search/Payments/Performance Loop

Reframe from "10 modules" to the Build/Deploy/Assess three layers, fold in
the June 2026 GA wave (harness, Managed Knowledge Base, Web Search, Policy+
Guardrails, Agent Performance Loop, Failure Insights) and Payments preview.
Rework the §4.2 gap analysis into a tri-state table tracking how the
platform/framework boundary moved toward the platform. Weave the "harness"
feature-name collision into the intro/§3.1/conclusion as a motif. Correct the
Registry name to "AWS Agent Registry", add Seoul region + compliance, refresh
GA statuses, fix the references numbering, and add the new source URLs.
Re-render the harness-mapping and gateway-flow diagrams (ASCII-only labels).
Publication date/URL left frozen at 2026-04-12.

Author: bits-bytes-nn