chore: add dependabot config for letta-code updates

Track @letta-ai/letta-code for daily version updates. Scoped to only
this dependency to avoid noise from other packages.

👾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

.github/dependabot.yml

@@ -0,0 +1,12 @@
+version: 2
+updates:
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    allow:
+      - dependency-name: "@letta-ai/letta-code"
+    labels:
+      - "dependencies"
+    commit-message:
+      prefix: "chore(deps):"