Rename agent-queue/docs/gigafactory/ to docs/GIGAFACTORY/ and update every
reference (README, system-overview code-map, and all phase job specs). Add an
index README that lists the docs and points to the companion docs in
learning_ai_common_plat. Docs-only; no behavior change.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
