feat: add @automaker/platform package

- Extract automaker path utilities (getFeatureDir, etc.)
- Extract subprocess management (spawnJSONLProcess, spawnProcess)
- Extract security/path validation utilities

Provides platform-specific utilities for:
- Managing .automaker directory structure
- Spawning and managing child processes
- Path validation and security

Dependencies: @automaker/types

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

libs/platform/package.json

@@ -0,0 +1,21 @@
+{
+  "name": "@automaker/platform",
+  "version": "1.0.0",
+  "description": "Platform-specific utilities for AutoMaker",
+  "main": "dist/index.js",
+  "types": "dist/index.d.ts",
+  "scripts": {
+    "build": "tsc",
+    "watch": "tsc --watch"
+  },
+  "keywords": ["automaker", "platform"],
+  "author": "",
+  "license": "MIT",
+  "dependencies": {
+    "@automaker/types": "^1.0.0"
+  },
+  "devDependencies": {
+    "@types/node": "^22.10.5",
+    "typescript": "^5.7.3"
+  }
+}