docs: re-align the H2 2026 roadmap with the published MCP roadmap (2026-08-22) and official extensions

Problem

docs/inspector-roadmap-2026-h2.md (#1980) was written when the MCP roadmap could not be read by the authoring agent. Its §3 was built from the 2026-03-05 public roadmap and WG charters. The published roadmap at https://modelcontextprotocol.io/development/roadmap is now dated 2026-08-22 and is organised around five different priority areas, so several of our Track A items are stale or missing, and a number of the roadmap's cited issues have since closed.

What changed upstream

1. Agentic Messaging Primitives — server-initiated events (webhooks) and a composition review of Tasks / triggers / subscriptions/listen / progress. Tasks-into-core (SEP-2663) is now listed under "Beyond", not this period.
2. HTTP-Native Transport Unification — HTTP over stdio; caching (SEP-2549 ttlMs/cacheScope is Final, ETags next). SEP-2575 (stateless) and SEP-2567 (sessionless / state handles) are Final, so our "session lifecycle lane" describes a model the spec dropped.
3. Agent Identity and Enterprise-Ready Security — DPoP is a this-period deliverable (we had it 🔴), plus Workload Identity Federation, ID-JAG, RFC 8693 token exchange. Audit trails, gateways and config portability are no longer on the roadmap.
4. Improved Primitives — tools/call result-shape redesign (content vs structuredContent), progressive discovery, possible deprecation of audience/priority annotations; File Uploads WG moved to scoped file ops / range reads / hierarchical listing. Streamed and reference results are gone.
5. Improved SDK Developer Experience — the extension contract, and spec-generated SDKs validated by the conformance suite.

Server Cards, Interceptors and Primitive Grouping are not on the published roadmap.

Extensions

Official extensions are Apps, OAuth Client Credentials, Enterprise-Managed Authorization, Tasks and Skills. The Inspector supports all but OAuth Client Credentials (#1225 was closed only because v1 is frozen). The upstream client matrix under-lists us (no Apps or EMA check, Skills "Partial").

Proposed change

• Rewrite the sourcing note and §3 against the 2026-08-22 roadmap's five areas, with revised confidence markers.
• Add an Official extensions section with a per-client support table and a standing process (an extension-watch sweep modelled on sdk-watch) for keeping up as extensions are approved.
• Mark items that have already shipped, and update §5 sequencing, §6 and §7 accordingly.