M1. stdio transport for the MCP server

## Why

MCP is HTTP-only today (default port `6128`). The most common MCP client integration pattern (Claude Desktop, many IDE agents, sandboxed hosts) launches servers over **stdio**. HTTP-only is a hard compatibility wall and forces port management. With adoption growing across 60+ accounts, broadening client reach is the highest-leverage agent investment.

## Proposed behavior

- `--mcp-stdio` (or `--mcp --transport stdio`) starts the server on stdio; keep HTTP as an option.
- Document both transports in `docs/mcp.md`.

## Acceptance criteria

- Server usable from a stdio MCP client end-to-end.
- HTTP behavior unchanged.
- Tests cover transport selection.
- Docs updated.

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_Filed from the Agentic & Automation Roadmap (`docs/agentic-roadmap.md`), item **M1**, Wave 1. Priority P0._

Start by reading the MCP server behavior and CLI startup path, then review the proposed transport options in the issue. Use docs/mcp.md and the existing test layout to determine where transport selection is covered. Done means stdio works end-to-end, HTTP remains unchanged, transport selection is tested, and both transports are documented.

Written by the indexing model from the issue text.