fix(suspend): make zellij inhibit watcher resilient to local-only sessions

The previous watcher exited immediately whenever no SSH-spawned zellij
was present. That caused a start-rate-limit storm:

  .path triggers service (zellij dir non-empty)
   -> watcher exits because no SSH zellij
   -> service stops
   -> .path retriggers (zellij dir still non-empty)
   -> ... 5 starts in 10s, systemd stops the path unit
   -> no inhibitor ever again, even after you SSH in

Restructure so the watcher stays alive for the entire zellij socket
directory lifetime and acquires/releases its own systemd-inhibit lock
dynamically based on SSH-zellij presence:

* Watcher now polls and exits only when the zellij socket dir is empty,
  matching the .path's trigger condition so it never re-fires while
  zellij is alive.
* systemd-inhibit removed from ExecStart - watcher self-inhibits via a
  child 'systemd-inhibit ... sleep infinity' it can terminate on demand.
* StartLimitIntervalSec=0 on the service as belt-and-braces against
  any future regression of the cycle.

Recovery from the rate-limit hit:
  systemctl --user reset-failed zellij-inhibit-suspend.service zellij-inhibit-suspend.path
  systemctl --user daemon-reload
  systemctl --user restart zellij-inhibit-suspend.path

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