Document near-term report behavior

docs/internal/briefing.md

@@ -38,6 +38,14 @@ Outputs:
 Every registered composition entry has a builder. Unknown or unimplemented
 module IDs fail validation instead of being skipped.
 
+Near-Term Report supports source and valid-period modules that operate over its
+rolling six-hour period: `metadata`, `current_conditions`, `hourly_forecast`,
+`precip_timing`, `alert_digest`, `spc_convective_outlooks`,
+`area_forecast_discussion`, `spc_convective_discussion`, and `weather_story`.
+It does not support daily/daypart-only modules such as
+`derived_daily_summary`, `derived_daypart_summaries`, `outdoor_windows`, or
+`tomorrow_planning`.
+
 Prompt-facing module values use local, human-readable date and time labels
 where the LLM is expected to reason about report content. Canonical timestamps
 remain in report metadata, source provenance, and integration artifacts.
@@ -59,7 +67,8 @@ builders run. Configured `location` values are prompt context only; Weather API
 `sourceLocationId` and `sourceLocation` remain source provenance.
 
 `area_forecast_discussion` uses optional `sections` configuration to include a
-subset of discussion fields.
+subset of discussion fields. Near-Term Report defaults this module to
+`key_messages` and `short_term`.
 
 `spc_convective_outlooks` uses collected SPC run metadata and derived
 report-period outlooks. It emits `checked: true` for a successfully fetched