feat: create @workflow/builders package with shared builder infrastructure

This commit extracts builder infrastructure from @workflow/cli into a new
shared @workflow/builders package. This improves code organization by:

- Creating a dedicated package for builder functionality
- Allowing @workflow/next and @workflow/nitro to depend on builders directly
- Reducing coupling between framework integrations and the CLI
- Preparing for moving NextBuilder to @workflow/next in the next PR

Changes:
- Created new @workflow/builders package
- Moved BaseBuilder, BasicBuilder, and VercelBuildOutputAPIBuilder
- Moved esbuild plugins (swc, discover-entries, node-module)
- Moved WorkflowConfig and BuildTarget types
- Updated @workflow/cli to import from @workflow/builders
- Updated @workflow/nitro to import from @workflow/builders
- Re-exported types from CLI for backwards compatibility

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: pranaygp

.changeset/create-builders-package.md

@@ -0,0 +1,7 @@
+---
+"@workflow/builders": patch
+"@workflow/cli": patch
+"@workflow/nitro": patch
+---
+
+Create @workflow/builders package with shared builder infrastructure

packages/builders/README.md

@@ -0,0 +1,43 @@
+# @workflow/builders
+
+Shared builder infrastructure for Workflow DevKit. This package provides the base builder class and utilities used by framework-specific integrations.
+
+## Overview
+
+This package contains the core build logic for transforming workflow source files into deployable bundles. It is used by:
+
+- `@workflow/cli` - For standalone/basic builds
+- `@workflow/next` - For Next.js integration
+- `@workflow/nitro` - For Nitro/Nuxt integration
+
+## Key Components
+
+- **BaseBuilder**: Abstract base class providing common build logic
+- **Build plugins**: esbuild plugins for workflow transformations
+- **SWC integration**: Compiler plugin integration for workflow directives
+
+## Usage
+
+This package is typically not used directly. Instead, use one of the framework-specific packages that extend `BaseBuilder`:
+
+```typescript
+import { BaseBuilder } from '@workflow/builders';
+
+class MyBuilder extends BaseBuilder {
+  async build(): Promise<void> {
+    // Implement builder-specific logic
+  }
+}
+```
+
+## Architecture
+
+The builder system uses:
+
+1. **esbuild** for bundling and tree-shaking
+2. **SWC** for transforming workflow directives (`"use workflow"`, `"use step"`)
+3. **Enhanced resolve** for TypeScript path mapping
+
+## License
+
+MIT

packages/builders/package.json

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+{
+  "name": "@workflow/builders",
+  "version": "4.0.1-beta.3",
+  "description": "Shared builder infrastructure for Workflow DevKit",
+  "type": "module",
+  "main": "./dist/index.js",
+  "types": "./dist/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./dist/index.d.ts",
+      "default": "./dist/index.js"
+    },
+    "./base-builder": {
+      "types": "./dist/base-builder.d.ts",
+      "default": "./dist/base-builder.js"
+    }
+  },
+  "files": [
+    "dist"
+  ],
+  "publishConfig": {
+    "access": "public"
+  },
+  "license": "MIT",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/vercel/workflow.git",
+    "directory": "packages/builders"
+  },
+  "scripts": {
+    "build": "tsc",
+    "clean": "tsc --build --clean && rm -r dist ||:",
+    "dev": "tsc --watch",
+    "test": "vitest run src",
+    "typecheck": "tsc --noEmit"
+  },
+  "devDependencies": {
+    "@types/node": "catalog:",
+    "@workflow/tsconfig": "workspace:*"
+  },
+  "dependencies": {
+    "@swc/core": "1.11.24",
+    "@workflow/swc-plugin": "workspace:*",
+    "@workflow/errors": "workspace:*",
+    "@workflow/core": "workspace:*",
+    "builtin-modules": "^5.0.0",
+    "chalk": "^5.6.2",
+    "comment-json": "4.2.5",
+    "enhanced-resolve": "5.18.2",
+    "esbuild": "catalog:",
+    "find-up": "7.0.0",
+    "tinyglobby": "^0.2.14"
+  }
+}

packages/builders/src/apply-swc-transform.ts

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+import { transform } from '@swc/core';
+import { createRequire } from 'module';
+
+const require = createRequire(import.meta.filename);
+
+export type WorkflowManifest = {
+  steps?: {
+    [relativeFileName: string]: {
+      [functionName: string]: {
+        stepId: string;
+      };
+    };
+  };
+  workflows?: {
+    [relativeFileName: string]: {
+      [functionName: string]: {
+        workflowId: string;
+      };
+    };
+  };
+};
+
+export async function applySwcTransform(
+  filename: string,
+  source: string,
+  mode: 'workflow' | 'step' | 'client' | false,
+  jscConfig?: {
+    paths?: Record<string, string[]>;
+    // this must be absolute path
+    baseUrl?: string;
+  }
+): Promise<{
+  code: string;
+  workflowManifest: WorkflowManifest;
+}> {
+  // Determine if this is a TypeScript file
+  const isTypeScript = filename.endsWith('.ts') || filename.endsWith('.tsx');
+  const isTsx = filename.endsWith('.tsx');
+
+  // Transform with SWC to support syntax esbuild doesn't
+  const result = await transform(source, {
+    filename,
+    swcrc: false,
+    jsc: {
+      parser: {
+        syntax: isTypeScript ? 'typescript' : 'ecmascript',
+        tsx: isTsx,
+      },
+      target: 'es2022',
+      experimental: mode
+        ? {
+            plugins: [[require.resolve('@workflow/swc-plugin'), { mode }]],
+          }
+        : undefined,
+      ...jscConfig,
+    },
+    // TODO: investigate proper source map support as they
+    // won't even be used in Node.js by default unless we
+    // intercept errors and apply them ourselves
+    sourceMaps: false,
+    minify: false,
+  });
+
+  const workflowCommentMatch = result.code.match(
+    /\/\*\*__internal_workflows({.*?})\*\//s
+  );
+
+  const parsedWorkflows = JSON.parse(
+    workflowCommentMatch?.[1] || '{}'
+  ) as WorkflowManifest;
+
+  return {
+    code: result.code,
+    workflowManifest: parsedWorkflows || {},
+  };
+}