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, 19 insertions, 0 deletions

diff --git a/dot_config/systemd/user/zellij-inhibit-suspend.service b/dot_config/systemd/user/zellij-inhibit-suspend.service
new file mode 100644
index 0000000..9e9ab40
--- /dev/null
+++ b/dot_config/systemd/user/zellij-inhibit-suspend.service
@@ -0,0 +1,19 @@
+[Unit]
+Description=Hold a systemd-inhibit lock while zellij sessions exist
+Documentation=man:systemd-inhibit(1) man:zellij(1)
+# Independent of any graphical session: this is meant to run on
+# headless SSH-attached hosts too.
+
+[Service]
+Type=simple
+ExecStart=systemd-inhibit \
+  --what=sleep:idle:handle-lid-switch \
+  --who=zellij \
+  --why=Active\x20zellij\x20sessions \
+  --mode=block \
+  %h/.local/bin/zellij-inhibit-watcher
+# Don't auto-restart: the .path unit reactivates us on the next session.
+Restart=no
+
+[Install]
+WantedBy=default.target