Add MCP MVP server with stdio/http transports

[mmaelicke]

Summary

• add an MCP MVP server for gorun with stdio and streamable HTTP transports
• implement MCP tools/resources including combined run_tool (validate + create + start)
• extract shared run/spec/result service logic used by both REST API and MCP
• add gorun mcp serve command and MCP defaults in config
• add MCP auth tests and update README docs

Verification

• GOCACHE=$(pwd)/.gocache go test ./...

Batman utility belt MVP: only essential gadgets shipped in this PR.