Replace hand-rolled MCP implementation with github.com/modelcontextprotocol/go-sdk

[mattt]

emcee was built early on, before there were any 3rd-party dependencies for MCP. It was also my first interaction with MCP. As a result, there are a bunch of things wrong with the original implementation.

This PR rips all of that out and replaces it with the official Go SDK.

[Copilot]

Pull Request Overview

This PR replaces the hand-rolled MCP implementation with the official github.com/modelcontextprotocol/go-sdk package, addressing issues in the original custom implementation that was built before third-party dependencies were available.

• Complete replacement of custom MCP protocol implementation with official SDK
• Refactoring of OpenAPI tool registration into separate internal package
• Updated test integration to handle proper MCP handshake protocol

Reviewed Changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
mcp/transport_test.go	Removed custom transport tests (functionality now handled by SDK)
mcp/transport.go	Removed custom transport implementation
mcp/server_test.go	Removed custom server tests (functionality moved to SDK)
mcp/server.go	Removed custom server implementation
mcp/protocol.go	Removed custom protocol definitions
main_test.go	Updated integration test to perform proper MCP handshake sequence
internal/openapi.go	Added new OpenAPI tool registration functionality using SDK types
go.mod	Added dependencies for official SDK and JSON schema library
cmd/emcee/main.go	Refactored to use SDK server with OpenAPI tool registration

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