Implements an observer pattern where agents can register watchers
for conditions on their thread. When the condition is met, the
agent gets "nagged" on subsequent invocations until it explicitly
closes the todo.
Key components:
- TodoRegistry: thread-scoped watcher tracking with eyebrow state
- TodoUntil/TodoComplete payloads and system handlers
- HandlerMetadata.todo_nudge for delivering raised eyebrow notices
- Integration in StreamPump dispatch to check and nudge
Greeter now demonstrates the pattern:
1. Registers watcher for ShoutedResponse from shouter
2. On next invocation, sees nudge and closes completed todos
3. Includes nudge in LLM prompt for awareness
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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