Adds first-class daemon control (the #845 pain point: no clean way to stop a
runaway daemon). `codegraph stop [path]` stops the current/given project's
daemon (SIGTERM -> SIGKILL fallback, sweeps artifacts); `stop --all` stops every
daemon; `list`/`ps` shows running daemons (--json for scripts).
Discovery via a small self-healing registry: each daemon records its root under
~/.codegraph/daemons/ on start, removes it on graceful shutdown; readers prune
dead pids. Cross-platform by construction (files + process.kill). Validated live
on macOS, Linux (docker), and Windows (VM): registry unit 6/6 and real-daemon
stop/list 6/6 on each.
