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

Adds a new `xcodegen validate` command that validates the project spec
without generating a project.

Output is always a JSON object with three keys:
  - valid (bool): whether the spec passed all checks
  - errors (array): each with stage and message fields
  - warnings (array): same shape

This separates validation from generation and enables CI pipelines to
get machine-readable validation results. Supports comma-separated --spec
paths and all existing ProjectCommand flags (--no-env, --project-root).

Exit code is 1 when the spec has errors, 0 when valid.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: samuraisatoshi

Sources/XcodeGenCLI/Commands/ValidateCommand.swift

@@ -0,0 +1,99 @@
+import Foundation
+import PathKit
+import ProjectSpec
+import SwiftCLI
+import XcodeGenKit
+import Version
+
+class ValidateCommand: ProjectCommand {
+
+    init(version: Version) {
+        super.init(version: version,
+                   name: "validate",
+                   shortDescription: "Validate the project spec without generating a project")
+    }
+
+    // Fully override execute() so that parsing errors are also captured as JSON
+    override func execute() throws {
+        var specPaths: [Path] = []
+        if let spec = spec {
+            specPaths = spec.components(separatedBy: ",").map { Path($0).absolute() }
+        } else {
+            specPaths = [Path("project.yml").absolute()]
+        }
+
+        var allErrors: [ValidationIssue] = []
+        var allWarnings: [ValidationIssue] = []
+
+        for specPath in specPaths {
+            guard specPath.exists else {
+                allErrors.append(ValidationIssue(stage: "parsing",
+                                                 message: "No project spec found at \(specPath)"))
+                continue
+            }
+
+            let specLoader = SpecLoader(version: version)
+            let variables: [String: String] = disableEnvExpansion ? [:] : ProcessInfo.processInfo.environment
+
+            let project: Project
+            do {
+                project = try specLoader.loadProject(path: specPath, projectRoot: projectRoot, variables: variables)
+            } catch {
+                allErrors.append(ValidationIssue(stage: "parsing", message: error.localizedDescription))
+                continue
+            }
+
+            do {
+                try specLoader.validateProjectDictionaryWarnings()
+            } catch let e as SpecValidationError {
+                allWarnings += e.errors.map { ValidationIssue(stage: "validation", message: $0.description) }
+            } catch {
+                allWarnings.append(ValidationIssue(stage: "validation", message: error.localizedDescription))
+            }
+
+            do {
+                try project.validateMinimumXcodeGenVersion(version)
+                try project.validate()
+            } catch let e as SpecValidationError {
+                allErrors += e.errors.map { ValidationIssue(stage: "validation", message: $0.description) }
+            } catch {
+                allErrors.append(ValidationIssue(stage: "validation", message: error.localizedDescription))
+            }
+        }
+
+        let result = ValidationResult(valid: allErrors.isEmpty, errors: allErrors, warnings: allWarnings)
+        stdout.print(try result.jsonString())
+
+        if !result.valid {
+            throw ValidationFailed()
+        }
+    }
+
+    // Not called — execute() is fully overridden above
+    override func execute(specLoader: SpecLoader, projectSpecPath: Path, project: Project) throws {}
+}
+
+// MARK: - JSON output types
+
+private struct ValidationIssue: Encodable {
+    let stage: String
+    let message: String
+}
+
+private struct ValidationResult: Encodable {
+    let valid: Bool
+    let errors: [ValidationIssue]
+    let warnings: [ValidationIssue]
+
+    func jsonString() throws -> String {
+        let encoder = JSONEncoder()
+        encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
+        let data = try encoder.encode(self)
+        return String(data: data, encoding: .utf8)!
+    }
+}
+
+private struct ValidationFailed: ProcessError {
+    var message: String? { nil }
+    var exitStatus: Int32 { 1 }
+}

Sources/XcodeGenCLI/XcodeGenCLI.swift

@@ -17,6 +17,7 @@ public class XcodeGenCLI {
                 generateCommand,
                 CacheCommand(version: version),
                 DumpCommand(version: version),
+                ValidateCommand(version: version),
             ]
         )
         cli.parser.routeBehavior = .searchWithFallback(generateCommand)