docs(3.1-beta): switch docs banner to beta + WG-review clarifications

[bokelley]

Two coordinated updates ahead of the 3.1 beta drop:

1. Docs banner switch

`docs.json` banner: 🎉 AdCP 3.0 is now GA → 🚀 AdCP 3.1 beta is now available, links to `/docs/reference/whats-new-in-3-1`. The 3.0 GA banner had been displayed on docs.adcontextprotocol.org for months and was out of date.

Intentionally not touched: `server/public/index.html` (AAO main site banner stays on "3.0 GA"). Different audience — operators, agencies, members. Beta messaging adds confusion there without value.

Other "3.0 GA" mentions in body docs (faq.mdx, design-principles.mdx, v2-sunset.mdx, etc.) are factual historical context and stay.

2. `whats-new-in-3-1.mdx` catch-up

• Beta status callout at top: spec feature-complete; SDK + grader advisory-only during beta; GA target 2026-05-29; adopters can pin `adcp_version: "3.1-beta"` today and migrate to `"3.1"` on GA without code changes.
• New "Final-spec clarifications (WG-review batch)" section covering the 10 normative tightenings from PR #4796 (`4c124545f1`) that landed after the original whats-new page was written:
  • `PROPOSAL_NOT_FOUND` error code (spec(errors): add PROPOSAL_EXPIRED and PROPOSAL_NOT_FOUND to canonical error catalog #4043)
  • Forward-compatible `error.code` decoding (spec(errors): make error.code forward-compatible decoding normative (unblock additive-in-patch for future maintenance lines) #4227) — open-enum receiver contract, recovery-classification rule
  • `idempotency_key` required on every AdCP task request (get_products: MCP envelope idempotency_key rejected by wrapper layer #4399)
  • MCP tool wrappers MUST tolerate envelope fields (get_products: MCP envelope idempotency_key rejected by wrapper layer #4399)
  • MCP serialization normalization, drop `payload.required`, add `context` envelope field (spec: clarify MCP serialization of core/protocol-envelope (payload nesting, context_id vs context echo) #2911)
  • Idempotency replay returns historical snapshot (Clarify idempotency-replay semantics for stateful resources (create_media_buy, sync_accounts) #4371)
  • `refine[]` finalize-exclusivity + multi-finalize atomicity (get_products refine[]: clarify mixed-action semantics when finalize is included #4107)
  • `pending_creatives` status disambiguation (Ambiguous status name: pending_creatives should clarify buyer action required #4196)
  • `notices` advisory channel on runner-output-contract (Standardize notices field on runner-output-contract (warn-on-skip / forward-readiness surface) #4418)

Test plan

• `test:docs-nav` ✓
• `git diff --stat origin/main..HEAD` shows only 2 files modified + 1 changeset (no stale-tree surprises)
• CI green

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