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/src/index.ts

@@ -0,0 +1,36 @@
+/**
+ * @automaker/platform
+ * Platform-specific utilities for AutoMaker
+ */
+
+// Path utilities
+export {
+  getAutomakerDir,
+  getFeaturesDir,
+  getFeatureDir,
+  getFeatureImagesDir,
+  getBoardDir,
+  getImagesDir,
+  getContextDir,
+  getWorktreesDir,
+  getAppSpecPath,
+  getBranchTrackingPath,
+  ensureAutomakerDir,
+} from './paths';
+
+// Subprocess management
+export {
+  spawnJSONLProcess,
+  spawnProcess,
+  type SubprocessOptions,
+  type SubprocessResult,
+} from './subprocess';
+
+// Security
+export {
+  initAllowedPaths,
+  addAllowedPath,
+  isPathAllowed,
+  validatePath,
+  getAllowedPaths,
+} from './security';