docs: propose A2A server architecture

[Tsuyoshi Ushio (TsuyoshiUshio)]

Purpose

Propose A2A server support through FRD 0009, a supporting architecture/design document, and a reviewer-sized implementation plan. Add the FRD index entry. This PR contains English documentation only; there are no production behavior, package dependency, or configuration changes.

The proposal selects Microsoft Agent Framework's agent-framework-hosting-a2a conversion helpers with application-owned lifecycle/session/authentication and a public SDK request-handler boundary. Alpha adoption and version/response-bridge compatibility remain explicit implementation gates.

Delivery prioritizes a working non-streaming server and sample for design validation, followed by basic SSE: D0 → P3 → P4 → P5a → P5b → P1 → P2 → P6a → P6b → P6c → P7 → P8 → P9a → P9b → P10. Existing Durable dependency/import compatibility work follows the working samples, with import isolation and optional extras/sample migration in separate PRs. Later slices cover atomic shared task/event storage, admission, managed execution, distributed subscriptions, cancellation, continuation, and qualification.

Does this introduce a breaking change?

• Yes
• No

Pull Request Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation content changes
• Other... Please describe:

How to Test

Review docs/frds/0009-a2a-server.md, docs/design/a2a-server.md, and docs/design/a2a-implementation-plan.md, starting with the FRD requirements and decisions log. Local links, fenced blocks, whitespace, and exact documentation-only scope were checked; git diff --check HEAD^ HEAD passes. No product test suite is required for this documentation-only PR.

What to Check

• Requirements, API mapping, MAF integration boundaries, and staged acceptance criteria agree across the three documents.
• P3-first sequencing supports a runnable sample and discussion before dependency optionalization.
• Basic streaming stays local-development-only with required enforcement; distributed capability exposure waits for lifecycle and qualification gates.
• Durable optionalization preserves workflow consumers and provides clean installation/import/startup guarantees only after the migration slice.

Other Information

FRD status remains In review. Documentation architecture review is complete, but human FRD implementation sign-off is still pending; this draft PR does not authorize or contain P3 implementation.

Japanese translations remain local review-only files and are not included. This is the bottom documentation PR for a prospective native GitHub stack. P3 will be a separate PR above it; no stack registration is performed until the second PR exists.