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>

apps/ui/tests/worktree-integration.spec.ts

@@ -103,9 +103,10 @@ test.describe("Worktree Integration Tests", () => {
     const branchLabel = page.getByText("Branch:");
     await expect(branchLabel).toBeVisible({ timeout: 10000 });
 
-    // Verify main branch button is displayed
-    const mainBranchButton = page.getByRole("button", { name: "main" });
-    await expect(mainBranchButton).toBeVisible({ timeout: 10000 });
+    // Wait for worktrees to load and main branch button to appear
+    // Use data-testid for more reliable selection
+    const mainBranchButton = page.locator('[data-testid="worktree-branch-main"]');
+    await expect(mainBranchButton).toBeVisible({ timeout: 15000 });
   });
 
   test("should select main branch by default when app loads with stale worktree data", async ({