Vertical-slice colocation: all main-agent code should live under
main_agent/ instead of being split across a parallel middleware/main_agent
tree. Move multi_agent_chat/middleware/main_agent/ -> main_agent/middleware/
and its assembler middleware/stack.py -> main_agent/middleware/stack.py, so
the main-agent slice is self-contained (graph, runtime, system_prompt, tools,
middleware).
Genuinely cross-slice middleware (middleware/shared/, middleware/subagent/)
stays under multi_agent_chat/middleware/ for a later slice; the moved builders
now reference it via absolute imports.
Pure move + import rewrite (git-tracked renames). Verified: full unit suite
green (2430 passed, 1 skipped), including test_import_all and the
checkpointed-subagent middleware suite.
9c845d562e