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.

3 files changed, 1 insertions, 38 deletions

diff --git a/dot_config/systemd/user/swayidle.service b/dot_config/systemd/user/swayidle.service
index 478c8f8..acd0196 100644
--- a/dot_config/systemd/user/swayidle.service
+++ b/dot_config/systemd/user/swayidle.service
@@ -10,6 +10,7 @@ ExecStart=/usr/bin/swayidle -w \
     timeout 300 'swaymsg "output * power off"' \
         resume  'swaymsg "output * power on"' \
     timeout 330 'swaylock -f -e -c 000000' \
+    timeout 1800 '%h/.local/bin/on-battery-suspend' \
     before-sleep 'playerctl -a pause; swaylock -f -e -c 000000' \
     lock         'swaylock -f -e -c 000000'
 Restart=on-failure
diff --git a/dot_config/systemd/user/zellij-inhibit-suspend.path b/dot_config/systemd/user/zellij-inhibit-suspend.path
deleted file mode 100644
index 2a4be21..0000000
--- a/dot_config/systemd/user/zellij-inhibit-suspend.path
+++ /dev/null
@@ -1,15 +0,0 @@
-[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.
-# The service's watcher then decides whether to actually hold the lock
-# (only if at least one zellij was spawned from an SSH session); if not,
-# it exits immediately and the service stops with no harm done.
-DirectoryNotEmpty=%t/zellij
-Unit=zellij-inhibit-suspend.service
-
-[Install]
-WantedBy=default.target
diff --git a/dot_config/systemd/user/zellij-inhibit-suspend.service b/dot_config/systemd/user/zellij-inhibit-suspend.service
deleted file mode 100644
index 7c73c64..0000000
--- a/dot_config/systemd/user/zellij-inhibit-suspend.service
+++ /dev/null
@@ -1,23 +0,0 @@
-[Unit]
-Description=Stay alive while any zellij session exists; inhibit suspend if SSH-spawned
-Documentation=man:systemd-inhibit(1) man:zellij(1)
-# Independent of any graphical session: this is meant to run on
-# headless SSH-attached hosts too. The watcher itself decides whether
-# the current zellij activity warrants inhibiting (SSH-spawned only),
-# and acquires/releases its own systemd-inhibit lock dynamically. It
-# stays alive for the whole zellij dir lifetime so the .path unit does
-# not retrigger us in a busy loop when only local zellij sessions are
-# active.
-# Disable systemd's default start-rate limiter: even though the
-# refactored watcher should not cycle anymore, a zero rate-limit makes
-# this unit resilient if the user kills it manually.
-StartLimitIntervalSec=0
-
-[Service]
-Type=simple
-ExecStart=%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