licence: Apache-2.0, one licence for everything

Apache-2.0 for all repository content, code and prose alike. Rationale: Mathlib compatibility upstream; §5 makes contributions inbound-equals-outbound, which matters for anonymous contributors; split licensing rejected as a per-file question that never ends. No per-file headers — the root LICENSE governs. Any copyright line reads "the alignment-workspace contributors", with no personal names.