feat: vscode client should only add code-workspace

[christso]

Why

when vscode is set as the client, it should not affect the location of plugin artifacts, especially skills.
vscode is a special case. It's basically a wrapper on copilot client.

Brainstorm

An alternative we can consider is vscode does not affect anything at all. It just adds the code-workspace.
That alternative might be a lot simpler and we don't need so many specs.
I want you to weigh both approaches - simply adding .code-workspace vs custom behaviour of choosing between .agents and .github (which might be difficult to reason).
However, that would be inconsistent with how we define clients. It is unintuitive that vscode does not add the required plugins, even if it does the same as copilot (with addition of code-workspace).

Once we agree on an approach then we can implement.

Specs

• When universal and vscode are set as clients, then use .agents (not .github)
• When none are set and only vscode is set as the client, then use .agents
• When universal and copilot and vscode are set as clients, then use .agents with symlink from .github. (existing behaviour)
• When copilot and vscode are set as clients (but not universal), then use .github (existing behaviour)
• This does not affect behaviour of subfolder .github, .claude etc under the plugin folder as those will get copied to .github, .claude etc as usual since they are client-specific overrides.