docs(brainstorming): correct Copilot CLI backgrounding guidance for Windows

[arimu1]

Who is submitting this PR? (required)

Field	Value
Your model + version	Gemini 2.0 Pro
Harness + version	Antigravity 2.0
All plugins installed	superpowers, caveman
Human partner who reviewed this diff	@arimu1

What problem are you trying to solve?

GitHub Copilot CLI on Windows uses PowerShell as its primary shell environment. This environment does not support bash-specific async tool options (bash tool, read_bash, stop_bash, mode: "async"). However, skills/brainstorming/visual-companion.md incorrectly advises Windows/Copilot CLI users to use these nonexistent bash-specific options.

This mismatch was originally reported in #1929.

What does this PR change?

This PR updates the Copilot CLI section in visual-companion.md to guide users to use the harness's general background execution mechanism instead of bash-specific options, aligning it with the general "Other environments" instructions in the same file.

Is this change appropriate for the core library?

Yes, it corrects general documentation for the core brainstorming skill.

What alternatives did you consider?

We considered adding .ps1 backgrounding scripts or native PowerShell adapters, but decided a docs-only fix pointing to the platform's general background execution mechanism is much simpler, safer, and follows the existing "Other environments" precedent.

Does this PR contain multiple unrelated changes?

No.

Existing PRs

• I have reviewed all open AND closed PRs for duplicates or prior art
• Related PRs: Small behavior fix in superpowers (#1929) #1973 (closed, unrelated whitespace formatting patch)

Environment tested

N/A (documentation-only correction).