feat(tmux-util): add ls subcommand

tmux-util ls is an opinionated replacement for `tmux ls` with columns for state (attached/detached), name, idle time (humanized), window count, and the current pane's cwd (tilde-fied if it sits under $HOME).

The cwd query goes through `tmux display -p -t <session> '#{pane_current_path}'`, which returns the cwd of the active pane of the active window. That's close enough to "what the session is about" for a one-line summary.

Idle calculation reads `date +%s` by default and accepts an override via the TMUX_UTIL_NOW env var so tests can pin "now" to a known epoch.

12 new tests cover Normal cases (attached / detached, multiple sessions) and Boundary cases (no sessions, idle exactly at minute / hour / day boundaries, $HOME tilde). One existing dispatch test got reworked because the original stub target (`ls`) is no longer unimplemented. Total suite is 26 tests, all green.

The fake-tmux harness picked up two things along the way: real format-string parsing for `list-sessions -F` and a new handler for `display -p`. The state file format extended to include activity epoch, window count, and cwd, with sensible defaults for older 3-tuple test inputs so the reap tests keep passing untouched.

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