Add `primer.config.json` for dogfooding defaults

[digitarald]

Problem

Primer supports primer.config.json for setting areas, policies, and defaults — but the Primer repo itself doesn't use one. A daily dev iterating on the repo has no pre-configured areas or policies.

The repo has a clear two-area structure (CLI in src/ and VS Code extension in vscode-extension/) but this isn't declared anywhere.

Suggestion

Add a primer.config.json that declares:

{
  "areas": [
    {
      "name": "cli",
      "description": "TypeScript CLI — commands, services, TUI",
      "applyTo": ["src/**"]
    },
    {
      "name": "vscode-extension",
      "description": "VS Code extension — commands, views, webview",
      "applyTo": ["vscode-extension/**"]
    }
  ]
}

This enables primer instructions --areas to generate scoped instructions and serves as a real-world example of the config format.