refactor: implement ALLOWED_ROOT_DIRECTORY security and fix path validation

This commit consolidates directory security from two environment variables
(WORKSPACE_DIR, ALLOWED_PROJECT_DIRS) into a single ALLOWED_ROOT_DIRECTORY variable
while maintaining backward compatibility.

Changes:
- Re-enabled path validation in security.ts (was previously disabled)
- Implemented isPathAllowed() to check ALLOWED_ROOT_DIRECTORY with DATA_DIR exception
- Added backward compatibility for legacy ALLOWED_PROJECT_DIRS and WORKSPACE_DIR
- Implemented path traversal protection via isPathWithinDirectory() helper
- Added PathNotAllowedError custom exception for security violations
- Updated all FS route endpoints to validate paths and return 403 on violation
- Updated template clone endpoint to validate project paths
- Updated workspace config endpoints to use ALLOWED_ROOT_DIRECTORY
- Fixed stat() response property access bug in project-init.ts
- Updated security tests to expect actual validation behavior

Security improvements:
- Path validation now enforced at all layers (routes, project init, agent services)
- appData directory (DATA_DIR) always allowed for settings/credentials
- Backward compatible with existing ALLOWED_PROJECT_DIRS/WORKSPACE_DIR configurations
- Protection against path traversal attacks

Backend test results: 654/654 passing ✅

🤖 Generated with Claude Code

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

docker-compose.override.yml.example

@@ -2,9 +2,13 @@ services:
   server:
     volumes:
       # Mount your workspace directory to /projects inside the container
+      # Example: mount your local /workspace to /projects inside the container
       - /Users/webdevcody/Workspace/automaker-workspace:/projects:rw
     environment:
-      # Set workspace directory so the UI can discover projects
-      - WORKSPACE_DIR=/projects
-      # Ensure /projects is in allowed directories
-      - ALLOWED_PROJECT_DIRS=/projects
+      # Set root directory for all projects and file operations
+      # Users can only create/open projects within this directory
+      - ALLOWED_ROOT_DIRECTORY=/projects
+
+      # Optional: Set workspace directory for UI project discovery
+      # Falls back to ALLOWED_ROOT_DIRECTORY if not set
+      # - WORKSPACE_DIR=/projects