Add A2A adapter + manifest v3.0.0 + discovery tool

[a6b8]

MCP Agent Server: A2A + Manifest v3.0.0

Context: Memo 002-mcp-agent-server. Adds A2A protocol support as optional adapter alongside existing MCP server.

Changes:

• fromManifest() updated for v3.0.0 (schemas array instead of schemasDir)
• New public methods: getToolConfig(), callTool()
• A2A Adapter: Agent Card generator, message/send → MCP tools/call translation
• A2A Discovery Tool: optional built-in tool in Agent Loop
• Reference implementation with MCP + A2A
• Package exports: ./a2a subpath

Affected files:

• src/AgentToolsServer.mjs (updated)
• src/agent/AgentLoop.mjs (updated)
• src/a2a/A2AAdapter.mjs (new)
• src/a2a/AgentCardGenerator.mjs (new)
• src/a2a/index.mjs (new)
• tests/manual/reference-implementation-a2a.mjs (new)
• package.json (updated)