Editor integration

When stdin is not a TTY, sigit starts an ACP server over stdio. Editors launch the binary and exchange JSON-RPC messages through that process.

For a custom ACP client, configure the executable as sigit and leave its stdin, stdout, and stderr attached. stdout is reserved for protocol messages.

The getsigit/sigit repository  contains the server implementation and the session lifecycle details for new, loaded, and forked sessions.