Improve onboarding: validate command and error messages

[ldraney]

Context

New users currently have to understand the sop.json schema by reading docs. The two biggest onboarding gaps:

1. validate command (Add JSON schema validation for sop.json #21 tracks the schema, this is about UX): Run npx dev-sop-engine validate and get clear, actionable error messages — not just "invalid JSON" but "rule 'no-env' references validator 'validators/check.sh' which doesn't exist".
2. Generator error messages: When generation fails (missing files, bad paths, invalid config), the errors should point to the exact problem and suggest a fix.

Relates to

• Implement init command for project scaffolding #20 (init command — the other half of onboarding)
• Add JSON schema validation for sop.json #21 (JSON schema validation — the underlying mechanism)
• Lower barrier to entry for non-technical users #24 (lower barrier for non-technical users)

What good looks like

$ npx dev-sop-engine validate
✗ rules.no-env-writes.validator: file "validators/check-env.sh" not found
  → Expected at: ./sop/validators/check-env.sh
✗ skills.workflow.content_file: file "./skills/workflow.md" not found
✓ logging configuration valid
✓ mcp configuration valid

2 errors found. Fix them and run again.