feat(suspend): hold inhibit lock while any zellij session exists

The SSH-shell inhibitor in dot_zprofile is bound to the lifetime of the
login shell, so it disappears the moment the user detaches a zellij
session and disconnects — defeating the whole point of using zellij for
persistent remote work.

Add a user-scope path+service+watcher trio that ties the inhibit lock
to the existence of zellij sessions instead:

  - dot_local/bin/executable_zellij-inhibit-watcher
      Polls `zellij list-sessions --short` every 15s, exits when none
      remain. Override poll interval via $ZELLIJ_INHIBIT_POLL.

  - dot_config/systemd/user/zellij-inhibit-suspend.service
      Wraps the watcher in `systemd-inhibit --what=sleep:idle:handle-lid-switch
      --mode=block`. When the watcher exits, the service stops and the
      lock is released.

  - dot_config/systemd/user/zellij-inhibit-suspend.path
      Activates the service whenever $XDG_RUNTIME_DIR/zellij becomes
      non-empty (i.e. zellij creates its first session socket). Re-fires
      on every empty→non-empty transition.

Enable via systemd-units/user.txt (the .path unit; the service is
on-demand).

The existing SSH-shell inhibitor is kept as a backstop for non-zellij
remote sessions and is now documented as such.

VM (nix/vm.nix) deliberately not updated: the Ubuntu remote-dev VM
never suspends, so the inhibit machinery would be inert there.

1 files changed, 8 insertions, 5 deletions

diff --git a/dot_config/zsh/dot_zprofile b/dot_config/zsh/dot_zprofile
index 02991cb..a26209b 100644
--- a/dot_config/zsh/dot_zprofile
+++ b/dot_config/zsh/dot_zprofile
@@ -151,11 +151,14 @@ case $(uname -n) in
 esac
 
 # ── SSH: inhibit suspend/idle while connected ────────────────────────────────
-# A remote session is useless if the laptop suspends mid-command. Wrap the
-# login shell in `systemd-inhibit` so a lock is held for the entire SSH
-# session lifetime (lid close, idle timeout, swayidle, lid-switch handlers
-# all honour it). The lock is released the instant the shell exits, so a
-# clean disconnect restores normal power behaviour.
+# Backstop for SSH sessions that don't use zellij. (The dedicated
+# zellij-inhibit-suspend.path user unit already covers any host that has
+# at least one live zellij session — that one survives detach/disconnect,
+# which this in-shell inhibitor does not.)
+#
+# Wrap the login shell in `systemd-inhibit` so a lock is held for the
+# entire SSH session lifetime; the lock is released the instant the
+# shell exits.
 if [[ -n $SSH_CONNECTION && -z $__SSH_SUSPEND_INHIBITED ]] \
   && command -v systemd-inhibit >/dev/null 2>&1; then
   export __SSH_SUSPEND_INHIBITED=1