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, 20 insertions, 0 deletions

diff --git a/dot_local/bin/executable_zellij-inhibit-watcher b/dot_local/bin/executable_zellij-inhibit-watcher
new file mode 100755
index 0000000..0af20dd
--- /dev/null
+++ b/dot_local/bin/executable_zellij-inhibit-watcher
@@ -0,0 +1,20 @@
+#!/bin/sh
+# Block until no zellij sessions remain.
+#
+# Used as the ExecStart payload of zellij-inhibit-suspend.service: the
+# service wraps this script with `systemd-inhibit`, so the inhibit lock
+# is held for exactly the lifetime of this process. When the last zellij
+# session ends, this script exits 0, the service stops, and the lock is
+# released.
+#
+# A user-level `.path` unit re-activates the service whenever the zellij
+# socket directory becomes non-empty again, so the lock is automatically
+# reacquired on the next `zellij` invocation.
+set -eu
+
+poll=${ZELLIJ_INHIBIT_POLL:-15}
+
+while sessions=$(zellij list-sessions --short 2>/dev/null) &&
+  [ -n "$sessions" ]; do
+  sleep "$poll"
+done