Decide one pending proposal. APPROVER OR OWNER KEY ONLY: developer and viewer keys are refused, so the key that configures mandates cannot sign off their output. approve executes nothing; it writes one signed projected-value ledger entry (a re-pin proposal moves the mandate to the newer profile version instead). reject needs a reason: not-material, client-preference, timing, data-is-wrong, already-done, or other (with a note). Refused when the proposal is not pending, has expired, or the household changed since it was computed. An agent should only call this when the person it acts for has made the decision. Same as POST /api/v1/proposals/{id}/approve and /reject.