refactor: remove duplicate lightning layer — already merged upstream (#223)

Removes packages/agentmesh-integrations/agent-lightning/ (merged to microsoft/agent-lightning PR #478) and dead-code duplicates from agent-os/integrations/agent_lightning/. Retains __init__.py backward-compat shim. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Copilot