Eliminate the top-level multi_agent_chat/middleware/ package so each slice
owns its middleware (vertical-slice colocation):
- middleware/shared/ -> shared/middleware/ (cross-slice middleware)
- middleware/subagent/ -> subagents/shared/middleware/ (subagent stack)
- main_agent/middleware/ already colocated in Slice A
The moved shared/ subtree is internally consistent (all relative imports
stay within it), so only external absolute refs were rewritten. The
subagent stack's ..shared.* relatives were promoted to absolute paths to
the new shared/middleware/ location.
multi_agent_chat/ root is now: main_agent/, shared/, subagents/.
Verified: 2430 unit tests pass, 1 skipped (baseline unchanged).
