fix: avoid msvc repair for unrelated commands

[loonghao]

Summary

• make companion runtime environment preparation side-effect-free for unrelated commands
• skip missing or broken companions instead of auto-installing or repairing them during prepare
• add regression coverage for missing and already-installed companions

Validation

• vx cargo test -p vx-resolver
• vx cargo build -p vx
• local smoke: temp project with msvc in vx.toml + target/debug/vx.exe git --version

Closes #889