feat(api): wire MCP + A2A protocol endpoints into Nexo.API

[IanFrelinger]

Summary

PR-4 of the MCP/A2A series — the application-layer wiring, targeting the long-lived application/mcp-a2a integration branch (created from the src-side chain #263→#266 so the adapter code exists underneath; this PR''s diff touches only application/** per the layer-boundary rules; lands on master via coordinated merge once the src chain is in).

All four protocol directions become available on the API host, feature-flagged off by default.

Changes

• Program.cs: AddNexoMcpServer().WithHttpTransport() + AddNexoMcpClient + AddNexoA2AServer + AddNexoA2ATransport (before AddNexo() so its scheme registration joins the remote-transport composition); read-only repo tools (repo.fs.read/repo.fs.list) registered for MCP allowlisting — mutating tools deliberately not pre-registered; MapNexoMcpEndpoint() + MapNexoA2AEndpoints() after the security chain with named per-IP rate-limit policies (nexo-mcp/nexo-a2a, off by default). No AllowAnonymous anywhere.
• NexoApiKeyAuthMiddleware: protocol-path handling — /api/mcp, /api/a2a, and the spec-mandated root /.well-known/agent-card.json are protected on all verbs (the MCP SSE listen channel is a GET that the default mutating-verb scope ignored, and the root card lives outside /api entirely). Card anonymity is an explicit opt-in mirrored from Nexo:A2A:Server:AllowAnonymousAgentCard via the new NexoProtocolIngressOptions, keeping the middleware decoupled from adapter packages.
• AgentRegistryA2ACatalog: host-side INexoA2AAgentCatalog over IAgentRegistry — the single place the domain persona model meets the A2A descriptor (adapter can''t reference Core.Domain per layering).
• IngressCapabilitymcp/a2a keys, IngressCatalog operator-inventory rows, NexoMiddlewareIngressOptions rate-limit fields, appsettings.json explicit disabled defaults.
• Nexo.API.csproj: adapter project references (note in-file: Nexo.API is not packed by any workflow today; if that changes the non-packable A2A refs must be revisited).

Testing

Live smoke on the built host (DOTNET_ROLL_FORWARD=Major, this machine lacks the net8 ASP.NET runtime):

• Disabled defaults: /health 200 · POST /api/mcp 405 (unmapped) · /.well-known/agent-card.json 404 — surfaces fully dark
• Enabled + ApiKey auth: POST /api/mcp initialize 401 without key, 200 with key returning a spec-correct result (protocolVersion negotiated down to the client''s 2025-06-18, serverInfo: nexo, tools capability)
• Nexo.API builds clean with warnings-as-errors

[skip-prod-style] — ProdStyle endpoint tests live in src/Nexo.Tests.Infrastructure which this application-base PR cannot touch; they land in the follow-up src-side PR (ProdStyle suite + mcp-a2a-gate.yml + compose env rows).

Testing strategy (blast radius)

Change type	Proof
API host wiring / auth middleware	Live-host smoke above; in-proc ProdStyle suite lands in the src-side follow-up PR

• make kernel-coverage-gate — n/a (no src/ changes)
• make kernel-gate — n/a
• make test-prod-style — deferred to follow-up src PR per layer boundary (see [skip-prod-style])

Checklist

• Documentation updated (design doc rows landed with the src-side PRs; DEPLOYMENT env examples land with compose changes in the follow-up)
• No TODO or NotImplementedException left unresolved
• Breaking changes are documented (none — all surfaces off by default; auth posture only adds protection on new paths)

Release (only when this PR ships a versioned NuGet/GHCR release)

• Not a versioned release — skip

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