feat(generate): add --dry-run flag to diff project without writing

[samuraisatoshi]

Summary

Adds a --dry-run flag to the generate command that generates the Xcode project entirely in memory and prints a JSON diff of what would change — without writing any files to disk.

This is useful for:

• CI checks that want to verify the spec is clean without modifying the repo
• Reviewing what XcodeGen would change before committing a spec change
• Debugging spec changes in a safe, read-only mode

Changes

• GenerateCommand.swift: new --dry-run flag; when set, calls ProjectDiff and prints JSON then returns early
• ProjectDiff.swift (new): lightweight struct that compares an in-memory XcodeProj against an existing .xcodeproj on disk and captures added/removed/modified PBX object keys

JSON output example

{
  "added": ["ABC123", "DEF456"],
  "modified": ["GHI789"],
  "removed": []
}

Test plan

• Run xcodegen generate --dry-run on an existing project — verify JSON output, no files written
• Run on a project with no existing .xcodeproj — all keys should appear in added
• Existing tests pass: swift test

🤖 Generated with Claude Code