Miniagents

* some improvements to support Web Research example

* README.md finalized (readme TODOs are taken care of)

* agent call traceability added to exception logs
* more intuitive names are given to certain concepts

- make history messages by MarkdownHistoryAgent customizeable
- fix a bug in mutable_state and agent forking
- rename agent_aggregators module to aggregator_agents
- optionally allow receiving errors from agents as text messages instead of them being risen as errors (a draft of the feature)

* First RAG in explainer agent (self dev)
* History agents pass incoming messages through by default (return_full_history=True makes the return full history in the end instead)
* Rename `mutable_kwargs` parameter in `miniagent` decorator (as well as in miniagent's `fork()` method) to `mutable_state`
* Minor changes inspired by Cursor (VS Code)

* MINOR: make user / AI assistant chat example from README.md more robust