Add screen-style multi-session support and recursive split panes

[wfaderhold21]

Multi-session (screen-style):

• Replace the singleton reattach info file with a session registry under ~/.config/linkshell/sessions/.json. Each detached server registers its own id, pid, sockets, and token; linkshell ls lists live sessions (pruning dead pids) and linkshell -r <id> reattaches to a specific one. Bare linkshell (and linkshell new [name]) always starts a fresh server; linkshell -r with no id attaches the sole running session.

Recursive split panes:

• New layout.rs: a binary LayoutTree (Leaf/Split) with split/close/rotate and in-order rect tiling. Replace the fixed two-pane LayoutMode + [Option;2] model with tree + Vec-based panes/pane_sizes so any pane can be split any number of times, in either direction. Chat docking now splits a dedicated pane and undocking closes it (sibling reclaims the space).
• Keys: alt-\ split right, alt-- split down, alt-w close, alt-r rotate, alt-o focus next. Config actions: split_pane_right, split_pane_down, close_pane, rotate_split, focus_next_pane.

Docs (README, in-app help, CHANGELOG) updated.