[codex] expose xim tool deps for project indices

[Sunrisepeak]

Summary

• seed the official xim index into project .xlings.json when project-local/custom indices are used
• keep the existing project data exposure so hook-time xpm deps like xim:python can resolve without relying on system tools
• prepare mcpp 0.0.40 for the hotfix release

Validation

• mcpp build
• mcpp --version => 0.0.40
• MCPP=... bash tests/e2e/01_help_and_version.sh
• MCPP=... bash tests/e2e/52_local_path_namespaced_index.sh
• MCPP=... bash tests/e2e/58_preinstall_mcpp_deps_for_hooks.sh
• git diff --check