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/paths.ts

@@ -0,0 +1,91 @@
+/**
+ * Automaker Paths - Utilities for managing automaker data storage
+ *
+ * Stores project data inside the project directory at {projectPath}/.automaker/
+ */
+
+import fs from "fs/promises";
+import path from "path";
+
+/**
+ * Get the automaker data directory for a project
+ * This is stored inside the project at .automaker/
+ */
+export function getAutomakerDir(projectPath: string): string {
+  return path.join(projectPath, ".automaker");
+}
+
+/**
+ * Get the features directory for a project
+ */
+export function getFeaturesDir(projectPath: string): string {
+  return path.join(getAutomakerDir(projectPath), "features");
+}
+
+/**
+ * Get the directory for a specific feature
+ */
+export function getFeatureDir(projectPath: string, featureId: string): string {
+  return path.join(getFeaturesDir(projectPath), featureId);
+}
+
+/**
+ * Get the images directory for a feature
+ */
+export function getFeatureImagesDir(
+  projectPath: string,
+  featureId: string
+): string {
+  return path.join(getFeatureDir(projectPath, featureId), "images");
+}
+
+/**
+ * Get the board directory for a project (board backgrounds, etc.)
+ */
+export function getBoardDir(projectPath: string): string {
+  return path.join(getAutomakerDir(projectPath), "board");
+}
+
+/**
+ * Get the images directory for a project (general images)
+ */
+export function getImagesDir(projectPath: string): string {
+  return path.join(getAutomakerDir(projectPath), "images");
+}
+
+/**
+ * Get the context files directory for a project (user-added context files)
+ */
+export function getContextDir(projectPath: string): string {
+  return path.join(getAutomakerDir(projectPath), "context");
+}
+
+/**
+ * Get the worktrees metadata directory for a project
+ */
+export function getWorktreesDir(projectPath: string): string {
+  return path.join(getAutomakerDir(projectPath), "worktrees");
+}
+
+/**
+ * Get the app spec file path for a project
+ */
+export function getAppSpecPath(projectPath: string): string {
+  return path.join(getAutomakerDir(projectPath), "app_spec.txt");
+}
+
+/**
+ * Get the branch tracking file path for a project
+ */
+export function getBranchTrackingPath(projectPath: string): string {
+  return path.join(getAutomakerDir(projectPath), "active-branches.json");
+}
+
+/**
+ * Ensure the automaker directory structure exists for a project
+ */
+export async function ensureAutomakerDir(projectPath: string): Promise<string> {
+  const automakerDir = getAutomakerDir(projectPath);
+  await fs.mkdir(automakerDir, { recursive: true });
+  return automakerDir;
+}