feat: report the running daemon's version (status + version command)

[StuBehan]

Summary

Playback happens in the long-running stackvox serve daemon, so if the daemon isn't restarted after an upgrade it keeps running the old engine while the installed package moves ahead — and engine fixes silently don't take effect. There was no way to see this: the socket was only ever probed for existence, never asked its version. This adds a version query and surfaces client/daemon skew in stackvox status.

Changes

• {"command": "version"} daemon protocol request → replies with the running daemon's stackvox version.
• daemon.version() client helper returning (got_version, version_or_error).
• stackvox status now prints a skew warning when the running daemon's version differs from the installed package — e.g. daemon running 0.5.0, but 0.8.0 is installed — restart the daemon (…) to pick it up.
• Daemons that predate the command reply with an error and are treated as "unknown", so there are no false positives.

Testing

• pytest — adds a protocol-level version test, a daemon.version() client test, and a status skew-warning test; ruff check . clean.
• Manual: echo '{"command":"version"}' | nc -U ~/.cache/stackvox/daemon.sock returns the running daemon's version; stackvox status against a stale daemon prints the skew line.

Related issues

Complements StackOneHQ/stack-nudge#124 (restart the daemon on app update) — together they close the "new client, stale daemon" skew that motivated this.