Publish the roadmap for the next specification release (#3290)

Replace the March 2026 roadmap with the five priority areas the Core Maintainers and Working Groups settled on for the next specification release: agentic messaging primitives, HTTP-native transport unification and hardening, agent identity and enterprise-ready security, improved primitives, and improved SDK developer experience. Each area names the maintainers holding it and the deliverables agreed for this cycle, alongside how the roadmap affects SEP prioritization.

Also move the roadmap out of its single-page sidebar group into the Community tab's renamed Shaping the Protocol group, alongside the design principles and SEP guidelines it points contributors to.

No-Verification-Needed: docs-only content and nav update

:house: Remote-Dev: homespace