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, 81 deletions

diff --git a/dot_local/bin/executable_zellij-inhibit-watcher b/dot_local/bin/executable_zellij-inhibit-watcher
deleted file mode 100755
index 7537b36..0000000
--- a/dot_local/bin/executable_zellij-inhibit-watcher
+++ /dev/null
@@ -1,81 +0,0 @@
-#!/bin/sh
-# Stay alive while any zellij session exists; hold a systemd-inhibit
-# lock only while at least one of those zellij sessions was spawned from
-# an SSH context.
-#
-# Rationale: a zellij session started locally (e.g. from a sway terminal)
-# is the user actively sitting in front of the laptop — that should NOT
-# inhibit suspend. Only zellij sessions started while SSH'd in deserve
-# the lock, so the host stays awake across detach + disconnect but
-# normal local-attended suspend still works.
-#
-# Detection: zellij's daemonised server is exec'd by the client and
-# inherits the client's environment. Linux preserves that exec-time
-# environment in /proc/<pid>/environ for the life of the process, even
-# after the original SSH session is gone. So an "ssh-spawned" zellij is
-# one whose environ contains SSH_CONNECTION=.
-#
-# Lifecycle: the .path unit starts this script when the zellij socket
-# directory becomes non-empty. The script then polls and stays alive as
-# long as any zellij socket exists, so the .path unit never re-triggers
-# the service while zellij is up (which previously caused a start-rate
-# limit storm when only local zellij was around). When the last zellij
-# exits, this script exits, the service stops, and the .path resumes
-# watching for the next session.
-set -eu
-
-poll=${ZELLIJ_INHIBIT_POLL:-15}
-sock_dir="${XDG_RUNTIME_DIR:-/run/user/$(id -u)}/zellij"
-
-has_ssh_zellij() {
-  pids=$(pgrep -x zellij 2>/dev/null) || return 1
-  for pid in $pids; do
-    [ -r "/proc/$pid/environ" ] || continue
-    if tr '\0' '\n' <"/proc/$pid/environ" 2>/dev/null |
-      grep -q '^SSH_CONNECTION='; then
-      return 0
-    fi
-  done
-  return 1
-}
-
-any_zellij_socket() {
-  [ -d "$sock_dir" ] || return 1
-  found=$(ls -A "$sock_dir" 2>/dev/null || true)
-  [ -n "$found" ]
-}
-
-inhibit_pid=
-release_inhibit() {
-  pid=$inhibit_pid
-  inhibit_pid=
-  [ -n "$pid" ] || return 0
-  kill "$pid" 2>/dev/null || true
-  wait "$pid" 2>/dev/null || true
-}
-trap release_inhibit EXIT INT TERM
-
-inhibit_alive() {
-  [ -n "$inhibit_pid" ] || return 1
-  kill -0 "$inhibit_pid" 2>/dev/null
-}
-
-acquire_inhibit() {
-  if inhibit_alive; then return 0; fi
-  systemd-inhibit \
-    --what=sleep:idle:handle-lid-switch \
-    --who=zellij \
-    --why='Active SSH-spawned zellij sessions' \
-    --mode=block \
-    sleep infinity &
-  inhibit_pid=$!
-}
-
-while any_zellij_socket; do
-  if has_ssh_zellij; then
-    acquire_inhibit
-  else
-    release_inhibit
-  fi
-  sleep "$poll"
-done