Local MCP mode via `--mcp` flag

[JoshMock]

Experimental: run CLI as a local MCP server by adding an --mcp flag. Details TBD.

[Keesan12]

A local --mcp mode would be really handy, especially for debugging and editor integrations.

A couple of contract details that usually matter more than the flag itself:

• make the startup output machine-readable enough that a client can detect readiness without scraping human log lines
• expose a cheap health / capability probe early so editors can tell "process is up" from "server is actually ready"
• decide whether stdio is the default transport or whether you also want a local TCP mode for easier manual testing
• fail loudly if a command is unsupported in MCP mode instead of quietly downgrading behavior

We kept running into that in MartinLoop-style MCP tooling: the painful bugs were usually not launch bugs, they were "the process is running but the client cannot tell whether the lane is writable yet."