knowledge_search, memory_injection and scoped_model_fallback no longer
belong in the cross-agent kernel (app/agents/shared/middleware): they are
consumed only inside multi_agent_chat. Relocate each impl next to the
builder that uses it:
- knowledge_search.py -> multi_agent_chat/shared/middleware/ (genuinely
shared: its _render_priority_message feeds kb_context_projection, used by
both the main agent and the KB subagent)
- memory_injection.py -> multi_agent_chat/shared/middleware/ (beside its
memory.py builder)
- scoped_model_fallback.py -> multi_agent_chat/shared/middleware/resilience/
(beside fallback.py/bundle.py)
Impls moved verbatim (git rename). Builders/consumers now import the local
sibling; main_agent knowledge_priority imports the new shared path; shared
middleware barrel trimmed.
Tests: repoint imports; convert the knowledge_search monkeypatch targets
from brittle dotted-string form to object-based patching (monkeypatch.setattr
on the imported module), which is robust to import ordering. No behavior
change.