Why: U3 slice 2 ships admission budgets (runtime.* keys, lease semantics, yield/reacquire) and the host-side action matrix; the runbook is the single source for operators and records the versioned public-contract plan KTD13 requires.
Evidence: backend branch feature/collab-storage-u3-budget-authz (admission.go, action matrix in server.go) with admission/matrix tests green.
Impact: docs only.
Refs: proposals/2026-10-11-collaboration-storage-and-distribution.md KTD13/R8, section 5 U3.
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