refactor(suspend): gate suspend on AC, drop bespoke zellij inhibit

New, simpler suspend policy:

  AC plugged in   -> never auto-suspends (lid close ignored, idle no-op)
  On battery only -> lid close suspends, swayidle suspends at 30 min idle

This replaces the SSH/zellij-aware inhibit machinery with a rule that
matches the user's mental model: if you don't want the machine to
sleep, plug it in. Long-running tasks (builds, downloads, SSH
sessions, headless services) just need AC.

Changes:

* etc/systemd/logind.conf.d/20-lid-ac.conf: set
  HandleLidSwitchExternalPower=ignore so logind itself handles the AC
  case at the source. No userspace daemon, no race, no rate-limit risk.
* dot_local/bin/on-battery-suspend: tiny POSIX wrapper that exits 0
  when any /sys/class/power_supply/{AC,ADP}*/online == 1, else execs
  `systemctl suspend`.
* dot_config/systemd/user/swayidle.service: add
  `timeout 1800 on-battery-suspend`. Idle suspend now exists, but only
  when on battery.
* Delete zellij-inhibit-suspend.{path,service} + watcher script and
  remove the entry from systemd-units/user.txt. The .path
  re-trigger storm bug is moot because the whole mechanism is gone.

Manual suspends (sway XF86Sleep keybind, sway power submode `s`,
`systemctl suspend` over SSH) still always work regardless of AC --
explicit user intent wins.

Also drop /migrate-podman-to-btrfs.sh from .gitignore; the one-off
migration script has been deleted now that the user has switched their
podman storage to the btrfs driver.

On-host steps to apply:

  chezmoi apply -v
  systemctl --user daemon-reload
  systemctl --user reset-failed zellij-inhibit-suspend.service zellij-inhibit-suspend.path || true
  systemctl --user stop zellij-inhibit-suspend.path zellij-inhibit-suspend.service || true
  systemctl --user disable zellij-inhibit-suspend.path || true
  systemctl --user restart swayidle.service
  # logind drop-in is reloaded automatically by the etc deploy script.

Verify:

  systemctl status systemd-logind | grep -i lid
  loginctl show-session $XDG_SESSION_ID | grep -i lid
  # Unplug AC -> close lid -> should suspend.
  # Plug AC   -> close lid -> nothing happens.

1 files changed, 0 insertions, 6 deletions

diff --git a/systemd-units/user.txt b/systemd-units/user.txt
index 77ba788..009b2b3 100644
--- a/systemd-units/user.txt
+++ b/systemd-units/user.txt
@@ -18,9 +18,3 @@ wob.service
 
 # --- mail (overridden via drop-ins in dot_config/systemd/user/) ---
 protonmail-bridge.service
-
-# --- suspend inhibitor: keep host awake while any zellij session exists ---
-# Enable the .path unit; it activates the service on-demand when zellij's
-# socket dir becomes non-empty. The service wraps a poller in
-# systemd-inhibit and exits when no sessions remain, releasing the lock.
-zellij-inhibit-suspend.path