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/tsconfig.json

@@ -0,0 +1,20 @@
+{
+  "compilerOptions": {
+    "target": "ES2020",
+    "module": "commonjs",
+    "lib": ["ES2020"],
+    "types": ["node"],
+    "declaration": true,
+    "declarationMap": true,
+    "outDir": "./dist",
+    "rootDir": "./src",
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "forceConsistentCasingInFileNames": true,
+    "resolveJsonModule": true,
+    "moduleResolution": "node"
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", "dist"]
+}