Track deferred tool namespaces in world state

[copyberry]

Track deferred tool namespaces in world state

What changed

• Add the disabled-by-default deferred_tool_world_state feature, which exposes deferred tool namespaces and their descriptions to the model in a <tools> world-state section.
• Emit added and removed namespace updates as tool availability changes, bound the rendered context size, and persist nonempty state across thread resumes.
• Omit empty tool state while retaining deferred tool discovery through tool_search.

Testing

• Cover initial, unchanged, removed, recovered, empty, and resumed namespace state, along with description truncation and rendered-size limits.