feat: add temporal context and contextual help

docs/INSTITUTIONAL_CONTEXT_CONTRACT.md

@@ -8,6 +8,9 @@ The shared contract lives in `govoplan_core.core.institutional`.
 Core owns reference shapes and provider protocols only. It does not own shared
 Mandate, Service, Party, Decision, evidence, or geography tables. Domain modules
 own persistence and authorization; optional capabilities resolve the references.
+Interactive reads use the separate platform temporal-data context documented in
+`TEMPORAL_DATA_CONTEXT.md`; it never changes current authorization or supplies
+mutation dates.
 
 ## Envelope