Fix installer detection and prompt for prior deploys

Two fixes:
1. coexistence.sh: ccgs_has_opencode_prior now also checks for
   .opencode/agents/creative-director.md — recognizes prior deploys
   that lost their install-state.json or VERSION file
2. install.sh: safety prompt reads from stdin instead of /dev/tty —
   allows piped input (echo 'y' | bash install.sh ...) for non-interactive
   contexts like CI or agent-driven deploys

Author: Cipher-85

.opencode/install.sh

@@ -61,7 +61,7 @@ if [ -n "${target_arg:-}" ] && [ "$install_mode" = "opencode_clean" ]; then
       printf 'The installer will deploy alongside existing content.\n\n' >&2
       if [ "$dry_run" -eq 0 ]; then
         printf 'Continue? [y/N] ' >&2
-        read -r response </dev/tty || response="n"
+        read -r response || response="n"
         case "$response" in
           y|Y|yes|YES) ;;
           *) printf 'Aborted.\n'; exit 1 ;;

.opencode/lib/coexistence.sh

@@ -65,6 +65,7 @@ ccgs_has_codex_ccgs() {
 ccgs_has_opencode_prior() {
   [ -f "${target_root:-$PWD}/.opencode/install-state.json" ] && return 0
   [ -f "${target_root:-$PWD}/.opencode/VERSION" ] && return 0
+  [ -f "${target_root:-$PWD}/.opencode/agents/creative-director.md" ] && return 0
   return 1
 }