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, 12 insertions, 0 deletions

diff --git a/dot_config/systemd/user/zellij-inhibit-suspend.path b/dot_config/systemd/user/zellij-inhibit-suspend.path
new file mode 100644
index 0000000..72c509c
--- /dev/null
+++ b/dot_config/systemd/user/zellij-inhibit-suspend.path
@@ -0,0 +1,12 @@
+[Unit]
+Description=Activate suspend inhibitor whenever zellij has a live session
+
+[Path]
+# %t expands to $XDG_RUNTIME_DIR (typically /run/user/$UID); zellij keeps
+# its per-version session sockets under this directory. Whenever the dir
+# transitions from empty to non-empty, the service is (re)activated.
+DirectoryNotEmpty=%t/zellij
+Unit=zellij-inhibit-suspend.service
+
+[Install]
+WantedBy=default.target