#13864: fix(docs): update macOS VM installation instructions for clarity and VNC usage

Quick fix on the macOS VM installation as I just tried it myself. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Updates the macOS VM (Lume) install doc to clarify when the VNC window is opened by adding an explicit `lume run openclaw` step during Setup Assistant, and adjusts the OpenClaw install and headless run instructions. The main issue to address before merge is the new headless command that backgrounds the VM and discards all output, which makes failures hard to diagnose and is inconsistent with the earlier quick-path instructions. <h3>Confidence Score: 4/5</h3> - This PR is low risk (docs-only) but has one copy/paste command that can hinder troubleshooting. - Changes are limited to documentation and generally improve clarity, but the new headless run command backgrounds the VM and redirects all output to /dev/null, which can hide real startup errors for users following the guide. - docs/install/macos-vm.md <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->