[codex] Hide Desktop attach from main COMSOL guidance

[jiweiqi]

Summary

• Remove the fragile Java Shell/UIA Desktop attach workflow from the public README's primary COMSOL paths.
• Keep the bundled skill aware of Desktop attach, but mark it as an advanced fallback only for explicit small edits in an already-open ordinary Desktop session.
• Preserve shared-desktop as the recommended visible COMSOL workflow.

Validation

• git diff --check HEAD~1..HEAD
• uv run pytest could not start locally because this Mac lacks a Java runtime, so jpype1 failed to build before tests ran.
• ./.venv/bin/python -m pytest could not run because the existing venv does not have pytest installed.