chore(mcp): untrack .mcp.json — machine-local config, path leak

pty is a public repo; PR #46 committed a `.mcp.json` with a
machine-specific absolute path (`/Volumes/SSD/.../coord/bin/st`).
That leaked one of myobie's local paths into public history and the
file isn't portable to any contributor who clones — they don't have
that path on disk.

Matches the smalltalk + transplant conventions: `.mcp.json` is
machine-local config, kept out of git via `.git/info/exclude`
(per-machine, not in .gitignore so the policy doesn't get committed).

The on-disk file is preserved — the agent's running MCP server reads
it on next session start as before. Future contributors set up their
own `.mcp.json` per-machine.

History note: the absolute path is still in older commits on `main`.
That's a benign disclosure (one filesystem layout, no secrets), so
no history rewrite — only future commits stop carrying the leak.