Address momus review of OMARCHY_INSTALL_MODE work

- install.sh: use realpath to follow symlinks when deriving OMARCHY_PATH
  from script location. Logical pwd left dev-link symlink chains pointing
  at the wrong dir; realpath resolves to the underlying checkout.
- install/helpers/mode.sh: validate explicit OMARCHY_INSTALL_MODE values
  and exit on typos. Also export_legacy_mode_flags now UNSETS the
  contradictory legacy flag for each mode so unmigrated callers can't
  see both OMARCHY_CHROOT_INSTALL=1 and OMARCHY_ONLINE_INSTALL=true.
- install/helpers/errors.sh: retry uses "$OMARCHY_PATH/install.sh"
  and preserves OMARCHY_INSTALL_MODE explicitly; the previous hardcoded
  ~/.local/share/omarchy/install.sh broke package-mode retries.
- bin/omarchy-install-mode: soften the summary; the helper only knows
  the current env, not any persisted post-install state.

install/helpers/errors.sh

@@ -122,7 +122,7 @@ catch_errors() {
 
     case "$choice" in
     "Retry installation")
-      bash ~/.local/share/omarchy/install.sh
+      OMARCHY_INSTALL_MODE="$OMARCHY_INSTALL_MODE" bash "$OMARCHY_PATH/install.sh"
       break
       ;;
     "View full log")

install/helpers/mode.sh

@@ -4,6 +4,15 @@
 
 detect_install_mode() {
   if [[ -n ${OMARCHY_INSTALL_MODE:-} ]]; then
+    case $OMARCHY_INSTALL_MODE in
+      iso-chroot|online-package|online-git) ;;
+      *)
+        echo "Error: invalid OMARCHY_INSTALL_MODE=$OMARCHY_INSTALL_MODE" >&2
+        echo "       must be one of: iso-chroot, online-package, online-git" >&2
+        exit 1
+        ;;
+    esac
+    export OMARCHY_INSTALL_MODE
     return
   fi
 
@@ -22,15 +31,22 @@ install_mode_is() {
   [[ ${OMARCHY_INSTALL_MODE:-} == "$1" ]]
 }
 
-# Sets OMARCHY_CHROOT_INSTALL=1 and OMARCHY_ONLINE_INSTALL=true so callers
-# that still check the legacy vars keep working through the transition.
+# Synchronize the legacy vars to the canonical mode so callers that still
+# check OMARCHY_CHROOT_INSTALL / OMARCHY_ONLINE_INSTALL agree with us
+# (including unsetting contradictory ones).
 export_legacy_mode_flags() {
   case ${OMARCHY_INSTALL_MODE:-} in
     iso-chroot)
       export OMARCHY_CHROOT_INSTALL=1
+      unset OMARCHY_ONLINE_INSTALL
       ;;
     online-git)
       export OMARCHY_ONLINE_INSTALL=true
+      unset OMARCHY_CHROOT_INSTALL
+      ;;
+    online-package)
+      unset OMARCHY_CHROOT_INSTALL
+      unset OMARCHY_ONLINE_INSTALL
       ;;
   esac
 }