* Socket access control: process ancestry check + file permissions
Redesign socket control modes from (off, notifications, full) to
(off, cmuxOnly, allowAll):
- cmuxOnly (default): uses LOCAL_PEERPID + sysctl process tree walk to
verify the connecting process is a descendant of cmux. External
processes (SSH, other terminals) are rejected.
- allowAll: hidden mode accessible only via CMUX_SOCKET_MODE=allowAll
env var, skips ancestry check. Legacy "full"/"notifications" env
values map here for backward compat.
- off: disables socket entirely.
Security hardening:
- Server: chmod 0600 on socket after bind (owner-only access)
- CLI: stat() ownership check before connect (reject fake sockets)
Removes per-command allow-list (isCommandAllowed) — once a process
passes the ancestry check, all commands are available.
Includes migration for persisted UserDefaults values and env var
aliases (cmux_only, cmux-only, allow_all, allow-all).
* Add /sync-branch skill for submodule + main sync
