fix(pgpm): clear update cache after pgpm update

[yyyyaaa]

• Invalidate the update-check cache (~/.pgpm/cache/update-check.json) after a successful pgpm update, so subsequent commands don't re-prompt with a stale "update available" message
• Verify the PATH-resolved binary after install and warn users when npm install -g went to a different location than their shell resolves (common with nvm/fnm/volta/pnpm)
• Replace string-based version comparison (> operator) with proper semver.gt() in the update check — the string comparison silently fails for multi-digit versions (e.g., "3.10.0" >
  "3.9.0" is false in JS)