Add scaffold command for `primer.config.json`

[digitarald]

Problem

primer.config.json supports areas, policies, and defaults — but there's no command to scaffold one. A new user discovering the config format has to read docs and write it manually. Compare with primer eval --init which scaffolds primer.eval.json from analysis.

Suggestion

Add primer init --config (or primer generate config) that:

1. Runs analyzeRepo() to detect monorepo areas
2. Generates a primer.config.json with detected areas and sensible defaults
3. Uses safeWriteFile() with --force semantics

Example output:

{
  "areas": [
    { "name": "frontend", "description": "React frontend app", "applyTo": ["apps/web/**"] },
    { "name": "api", "description": "Express API server", "applyTo": ["packages/api/**"] }
  ],
  "policies": []
}

This mirrors the eval --init pattern and helps users discover the config format.