find_daemon() replaces daemon_sock() — walks the dot-delimited channel
path from most-specific to least looking for a daemon socket, and
auto-starts via the supervisor if none is found. All channel tools
(recv, send, open, close) use the same resolution path.
Fix tmux daemon to use pane_id consistently for both pipe-pane and
send-keys (send-keys -t <label> doesn't work, needs the %N pane id).
Store label→pane_id mapping in State instead of bare label vec.
Gracefully handle missing tmux.json5 — start with empty pane list
since panes are added dynamically via the open RPC.
Co-Authored-By: Proof of Concept <poc@bcachefs.org>