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.

bin/omarchy-install-mode

@@ -1,11 +1,8 @@
 #!/bin/bash
 
-# omarchy:summary=Print the install mode the installer was last invoked with
+# omarchy:summary=Print the OMARCHY_INSTALL_MODE in the current env
 # omarchy:group=install
 
-# Used by scripts that need to branch on whether Omarchy was bootstrapped
-# via ISO (iso-chroot), pacman install (online-package), or boot.sh
-# (online-git). The current install.sh sets this; persisted state lives in
-# /var/lib/omarchy/install-mode if Chunk 6+ chooses to record it. For now
-# we echo whatever is in the current env.
+# install.sh exports this during the install flow. Not persisted post-install,
+# so this prints 'unknown' outside the installer.
 echo "${OMARCHY_INSTALL_MODE:-unknown}"