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")