v0.5.0: Add an MCP server over the data tools

Mcp\McpServer speaks the Model Context Protocol (Streamable HTTP, the
stateless variant: one JSON-RPC POST, one application/json response), so
other agents can query the anonymized data the chat works with. It
publishes get_database_schema and execute_sql from any ToolExecutor as
read-only tools, runs every string of a tool result through
PiiResponseFilter, and logs each tool call. Authentication stays with
the caller; McpResponse carries status, headers and body plus the
401/403/405 helpers.