feat(init): --ci scaffolds ci workflow; add reusable workflow (#216)

[mojoatomic]

Implements #216:

• CLI: init --ci now scaffolds .github/workflows/ci-ratchet.yml (non-destructive; creates if missing)
  • Job: ratchet-and-tests (matches branch protection guidance)
  • Steps: checkout → setup-node@20 → npm ci → lint:json → analyze:current → ratchet → ratchet:context (health telemetry/gate) → upload artifacts
• Reusable workflow: .github/workflows/ratchet-reusable.yml (workflow_call) so consumers can uses: it instead of copying YAML
• Keeps existing package.json script injection (lint:json, analyze:current, ratchet, ratchet:context) from prior work

Notes

• Health gate will only fail when health.enabled=true in .ai-coding-guide.json; telemetry prints otherwise.
• Generator is conservative: if a workflow already exists at that path, it is left unchanged.

Docs to follow in #215 with both options (scaffolded file vs reusable workflow) and branch protection guidance.