acp: MCP server passthrough from IDE client

[bug-ops]

Parent: #762

Task

Accept MCP server configurations from the IDE client during initialization and register them with zeph-mcp.

Acceptance criteria

• Parse McpServer entries from InitializeRequest capabilities
• Support stdio, HTTP, and SSE MCP server types
• Register received MCP servers with McpManager
• Merge IDE-provided MCP servers with agent's own MCP config
• Report available MCP tools back to IDE via capabilities