Closes the usage-tracking half ADR 0050 left open. One row shape (0050's), recorded at two
consumer grains: the holding module (licence-level, e.g. Anthropic utilization%) and the agent
session (per-session tokens/cost, since a session IS a consumer per ADR 0026). Produced by the
vendor adapter; read on a schedule (0053); recorded as events the audit-logger keeps (0041/0042)
plus a queryable usage context store (0008); usage is not a credential and is recorded in the
clear. Reuses the session, schedule, event, and store the mesh already has. Accepted per the
user's choice to build the full feature incl. per-session token/cost.
Claude-Session: https://claude.ai/code/session_01LrgweAeERJYBg88c5cKDzF
jschoubben