Remove deprecated backend logic files

Deleted files no longer needed after migrating to Letta's native scheduling:

- letta_executor.py - Letta now handles execution
- scheduler.py - Letta now handles cron checking
- crypto_utils.py - No file storage needed
- models.py - Using Letta's API models
- test_api.py - Old API tests no longer relevant
- tests/ - Old test suite for deprecated backend

Result: Removed ~500 lines of deprecated code.

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

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

.gitignore

@@ -34,3 +34,4 @@ Thumbs.db
 
 # Test outputs
 *.log
+.skills/