Infer agent migration and new skills folder

[digitarald]

The changes (deprecate infer went in late in the release:

1. Deprecare infer
2. New .agents/skills folder support

We missed updating the customization skill, which now creates invalid custom agents and skills; and the plan agent, which means it can be called as a sub-agent, leading to odd side effects as it can use the ask questions tool.

• PR: [0.37] Infer agent migration and new skills folder vscode-copilot-chat#3463 (cherry-pick to release/0.37)[/+]
  [+]- Original PR: Infer agent migration and new skills folder vscode-copilot-chat#3464