feat(validate): add validate command with structured JSON output

[samuraisatoshi]

Summary

Adds a new xcodegen validate command that validates the project spec without generating a project, producing structured JSON output.

• Validates one or more specs (comma-separated --spec paths)
• Catches parsing errors, validateProjectDictionaryWarnings, validateMinimumXcodeGenVersion, and project.validate() — each attributed to a stage
• Outputs a JSON object with valid, errors, and warnings arrays
• Exits with code 1 when the spec has errors (so CI can gate on it)

JSON output

{
  "errors": [],
  "valid": true,
  "warnings": []
}

Error/warning objects:

{
  "errors": [
    { "stage": "parsing", "message": "..." },
    { "stage": "validation", "message": "Target 'Missing' not found" }
  ],
  "valid": false,
  "warnings": []
}

Use cases

• CI pre-check: validate spec before running generate, with machine-readable output
• Editor integration: parse JSON to show inline errors
• Multi-spec monorepos: --spec app/project.yml,lib/project.yml validates both, aggregates results

Test plan

• xcodegen validate on valid spec → "valid": true, exit 0
• xcodegen validate on invalid spec → "valid": false, errors populated, exit 1
• xcodegen validate --spec missing.yml → parsing error captured in JSON, exit 1
• xcodegen validate --spec a.yml,b.yml → errors from both specs aggregated
• swift test passes

🤖 Generated with Claude Code