* feat(mcp): add Model Context Protocol server support
Add MCP client and provider packages that allow integrating external
MCP tool servers into the review loop. Includes config commands for
managing MCP servers, stdio subprocess integration tests, and
comprehensive test coverage.
* refactor(mcp): rename loop variable in contentToText to avoid shadowing Client receiver
* fix(mcp): use platform-specific shell for setup command
The MCP server setup command was hardcoded to use `sh -c`, which
fails on Windows. Extract a `shellCommand` helper behind build tags
to use `cmd /c` on Windows and `sh -c` elsewhere.
* docs(mcp): add MCP server documentation to all README locales
