Ensure `__init__.py` exports are complete and correct

[dgenio]

Summary

Verify that agent_kernel/__init__.py exports all public types and that imports work.

Must export at minimum

• Kernel
• All models (Capability, CapabilityRequest, CapabilityGrant, Principal, Frame, Handle, etc.)
• All enums (SafetyClass, SensitivityTag, ResponseMode)
• All errors
• CapabilityRegistry, DefaultPolicyEngine, HMACTokenProvider, StaticRouter
• InMemoryDriver, HTTPDriver
• Firewall, HandleStore, TraceStore
• Budgets

Acceptance criteria

• from agent_kernel import Kernel, Capability, SafetyClass, ... works for all public types
• __version__ is set to "0.1.0"
• __all__ is defined and matches exports