[FEATURE] Implement Hooks

[edenreich]

Summary

Hooks are things we can execute when the agent finished generating. This is useful when we need deterministic output. Some might need it, some might not, so will keep it as a feature flag.

Acceptance Criteria

• There is a way to execute a arbitrary code when the agent finished generating
• It's extensible
• It's a feature flag, turned off by default
• It's documented
• It's tested