docs: add AGENTKIT.md for AgentKit APIs (#142)

* docs: add AGENTKIT.md for AgentKit APIs and link from README

* docs(agentkit): drop M2M from AGENTKIT.md

* docs(agentkit): add client init and clarify auth namespace

* docs(agentkit): remove MCP BYOA auth from AGENTKIT (see REFERENCE)

* docs(agentkit): remove table of contents from AGENTKIT

* docs: remove AGENTKIT link from README

* docs(agentkit): align list_scoped_tools filter param wording with SDK

* docs(agentkit): fix MD001 by using h3 for detail section labels

Author: saif-at-scalekit