fix(codex): preserve explicit memory directory in installer

[dommonkhouse]

Summary

• preserve an explicitly supplied MEMSEARCH_DIR in installed Codex hook commands
• keep the default installer behaviour unchanged when MEMSEARCH_DIR is not set
• quote the preserved path safely for hook command execution

Verification

• bash -n plugins/codex/scripts/install.sh
• installer smoke test with MEMSEARCH_DIR unset writes normal hook commands
• installer smoke test with MEMSEARCH_DIR set writes MEMSEARCH_DIR-prefixed hook commands
• git diff --check
• scanned diff additions for user-specific local paths

[dommonkhouse]

Ready for maintainer review and merge. I verified this PR is non-draft, mergeable, and has no configured checks reporting failures.