wire up coord channel: add .mcp.json registering `coord mcp --channel`
Repo-local MCP registration so Claude Code in this repo's pty
session boots with the coord channel-mode MCP server connected.
Identical contents to the coord repo's .mcp.json — coord is the
producer of the MCP server, pty just consumes it. Points at the
coord repo's `bin/coord` directly.
env is empty: COORD_IDENTITY=pty-claude is set in pty.toml (next
commit), COORD_ROOT defaults to ~/.local/state/coord.
Part of brief-018 (switchover package) — see
github.com/myobie/coord notes/brief-018-switchover-package.md.