feat(core): add middleware pipeline architecture with onion model
U6: Unified middleware protocol (before/after) with MiddlewareChain
implementing onion model execution. Parallel integration (KTD1) —
middleware path controlled by presence of middleware_chain parameter,
existing ReActEngine path unchanged when None.
- New core/middleware.py: RequestContext, Middleware protocol,
MiddlewareChain (onion model: before outer→inner, after inner→outer)
- 3 example middlewares: SummarizationMiddleware (U3 headroom compression),
TokenUsageMiddleware, LoopDetectionMiddleware (request-level audit)
- ReActEngine.__init__ accepts middleware_chain parameter
- execute() branches: middleware path when chain present, existing path otherwise
- 22 tests covering ordering, error handling, state passing, backward compat
3dfda904d7