Auto-detect and select Compass CLI versions

[forhappy]

Summary

• discover every executable from the configured path, PATH, and common platform install locations
• probe each installation with compass --version and display version, source, and full path in a quick pick
• keep the configured executable first while allowing users to switch versions or browse for another binary
• verify manually selected executables and require confirmation when their version cannot be read
• update command metadata, walkthrough copy, and VS Code setup documentation

Verification

• VS Code unit suite: 60 tests passed
• VS Code typecheck passed
• production extension build passed
• graphify update . completed