docs(copilot): clarify skill and persona installation

[anarchitect]

Summary

• keep the existing GitHub Copilot skill-installation guidance unchanged
• clarify that skills and custom-agent personas are separate customizations
• retain the existing copy pattern and add the fourth web-performance-auditor persona
• document current VS Code agent-picker, Copilot CLI, and cloud-agent usage

Why

The existing skill setup works, but it does not install the repository's agents/ personas. Users can therefore have working skills and still see no selectable custom agents. The previous guide also described @ invocation rather than the current agent-picker flow.

Scope

Documentation only. This PR changes only the Agent Personas section in docs/copilot-setup.md. It does not change skill installation, lifecycle slash commands, plugin manifests, or runtime behavior.

Verification

• original-style copy commands tested in a fresh temporary repository: four valid .agent.md files created
• copilot --agent code-reviewer verified with GitHub Copilot CLI 1.0.72-1
• node scripts/validate-skills.js
• node scripts/validate-commands.js
• git diff --check

[addyosmani]

Copilot is a supported platform, so a clearer skills-and-persona section in its setup doc is welcome. I didn't independently verify every Copilot-specific detail since you're closer to that surface, so I'd want one check that the paths and invocation match current Copilot behavior before it lands. Structure reads right otherwise.

[anarchitect]

Narrowed this PR after rechecking the installation boundary:

• restored the existing skills section unchanged;
• removed the persistent agent-skills-source path assumption;
• changed both persona-install examples to clone into a temporary directory and clean it up;
• verified the exact Bash and PowerShell flows each create all four valid .agent.md files;
• reverified copilot --agent code-reviewer on GitHub Copilot CLI 1.0.72-1.

The final diff now covers Copilot persona onboarding only.

[anarchitect]

Simplified the PR to the original copy pattern after review:

• removed the temporary-clone and PowerShell machinery;
• retained the existing three cp /path/to/agent-skills/... commands;
• added only the fourth web-performance-auditor copy line;
• clarified that the placeholder refers to an existing local clone;
• kept the corrected Copilot agent-picker, CLI, and cloud usage guidance.

The existing skill-installation section remains unchanged. The four copy commands were verified in a fresh repository.

[addyosmani]

The simplification is the right call, and thanks for actually verifying copilot --agent code-reviewer on 1.0.72-1, that's the check I was asking for. Adding the missing fourth persona copy line while leaving the existing skills section untouched is far easier to reason about than the temp-clone version. It's in draft, so flag me when you flip it and I'll look properly.

[addyosmani]

The direction is good. Softening the old "plain *.md files are silently ignored" claim to "VS Code detects them but other Copilot surfaces may not" is more accurate, and adding the CLI (copilot --agent, /agent) and cloud-agent paths fills real gaps in this doc. Picking up web-performance-auditor so all four personas are listed is a nice catch too.

Two things before this lands. Most of this is behavioral claims about how Copilot discovers agents across VS Code, CLI, and cloud, and those move fast; can you confirm which Copilot versions you verified these against, so we're not documenting a state that's already shifted? And CI hasn't actually run on this yet (checks are empty, mergeable state is unstable), so I'd want a green run before merging. Happy to take it once those two are settled.