dbui mcp: headless MCP server subcommand + shared session store

Same binary, `dbui mcp` (stdio): reuses the connection layer, config resolution, and safety rails. Both TUI and MCP mode write a shared session store (queries, timing, result metadata) so agents and the human share history and an audit trail — shared STORAGE, not a shared process. Tools: list sources/schemas/tables, describe, query (safety-classified), session history.

**Acceptance:** a coding agent can drive a configured source through `dbui mcp` with read-only enforced; TUI shows the agent's queries in history.

Start at the existing dbui command entry point and trace the connection layer, config resolution, safety rails, TUI history, and session storage. Define the dbui mcp stdio entry point and shared storage behavior across TUI and MCP modes, then verify the acceptance cases: read-only agent queries work and appear in TUI history with timing and result metadata.

Written by the indexing model from the issue text.