enhancement(mcp): add a --nightly flag to the MCP bootstrap flow

[captainsafia]

Reported during the March 13, 2026 nteract bug bash.

Current behavior

Using the nightly MCP requires manually wiring RUNTIMED_SOCKET_PATH, for example:

claude mcp add nteract-nightly -- env RUNTIMED_SOCKET_PATH="$HOME/Library/Caches/runt-nightly/runtimed.sock" uvx --prerelease allow nteract

Requested improvement

Support a dedicated --nightly flag so the bootstrap path is simpler, for example:

uvx --prerelease allow nteract --nightly

Why this matters

The current setup is easy to get wrong and adds friction to nightly testing.