Add SPC convective discussion briefing module

docs/internal/briefing.md

@@ -29,7 +29,8 @@ Outputs:
 - `module.Output` values for source-oriented stanzas:
   `metadata`, `current_conditions`, `narrative_forecast`, `hourly_forecast`,
   `alert_digest`, `spc_convective_outlooks`,
-  `area_forecast_discussion`, and `weather_story`
+  `area_forecast_discussion`, `spc_convective_discussion`, and
+  `weather_story`
 - `module.Output` values for derived stanzas:
   `derived_daily_summary`, `derived_daypart_summaries`, `precip_timing`,
   `outdoor_windows`, and `tomorrow_planning`
@@ -90,6 +91,8 @@ return an error for invalid required inputs.
 - SPC convective outlook output distinguishes checked empty outlook data from
   missing outlook source data and omits GeoJSON geometry from prompt-facing
   fields.
+- SPC convective discussion output is omitted unless a retained outlook meets
+  the internal severity threshold and matching discussion text is available.
 
 ## Tests