Recursive pass over the agents module to make docstrings and inline
comments concise and intent-oriented: drop narration that just restates
the code, condense verbose module/function docstrings, and keep only the
non-obvious "why" notes. No functional code changed.
Move the lower-level runtime/infra modules out of multi_agent_chat/shared/
(they were never used by subagents, so they failed the shared-by-all-siblings
rule) and unify them with the already-relocated checkpointer:
agents/runtime/ -> agents/chat/runtime/
mac/shared/errors.py -> chat/runtime/errors.py
mac/shared/llm_config.py -> chat/runtime/llm_config.py
mac/shared/prompt_caching.py -> chat/runtime/prompt_caching.py
mac/shared/mention_resolver.py -> chat/runtime/mention_resolver.py
mac/shared/path_resolver.py -> chat/runtime/path_resolver.py
These sit below the agent packages: the boundary + agent factory + shared
middleware depend on them, and they import no agent code (acyclic).
