fix(handoff): create .stackmemory directory before writing handoff file

Fixes ENOENT error when running handoff in a project without an existing
.stackmemory directory. Now creates the directory with recursive: true
before attempting to write last-handoff.md.

Author: jonathanpeterwu

src/cli/commands/handoff.ts

@@ -316,11 +316,11 @@ Generated by stackmemory handoff at ${timestamp}
         }
 
         // 7. Save handoff prompt (both latest and versioned)
-        const handoffPath = join(
-          projectRoot,
-          '.stackmemory',
-          'last-handoff.md'
-        );
+        const stackmemoryDir = join(projectRoot, '.stackmemory');
+        if (!existsSync(stackmemoryDir)) {
+          mkdirSync(stackmemoryDir, { recursive: true });
+        }
+        const handoffPath = join(stackmemoryDir, 'last-handoff.md');
         writeFileSync(handoffPath, handoffPrompt);
 
         // Save versioned copy