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.sh

@@ -6,7 +6,7 @@ set -eEo pipefail
 # whether it's run from /usr/share/omarchy/install.sh (package mode) or
 # $HOME/.local/share/omarchy/install.sh (git mode). An explicit OMARCHY_PATH
 # in the caller env still wins.
-_OMARCHY_INSTALLER_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+_OMARCHY_INSTALLER_DIR=$(dirname "$(realpath "${BASH_SOURCE[0]}")")
 export OMARCHY_PATH="${OMARCHY_PATH:-$_OMARCHY_INSTALLER_DIR}"
 export OMARCHY_INSTALL="$OMARCHY_PATH/install"
 export OMARCHY_INSTALL_LOG_FILE="/var/log/omarchy-install.log"