Add JSON Schema validation to akf validate command

[HMAKT99]

Summary

The akf validate command currently checks structural validity but doesn't validate against the official JSON Schema at spec/akf-v1.1.schema.json. Adding schema validation would make validation more thorough.

What needs to happen

1. Add jsonschema as an optional dependency
2. In python/akf/cli.py, enhance the validate command to also run JSON Schema validation
3. Report schema violations with clear error messages
4. Gracefully handle missing jsonschema package

Helpful context

• Schema is at spec/akf-v1.1.schema.json
• Current validation logic is in python/akf/validate.py
• Use jsonschema.validate() from the jsonschema package

Acceptance criteria

• akf validate file.akf reports schema violations
• Clear error messages for each violation
• Works without jsonschema installed (falls back to current behavior)
• Tests added