Implement source-oriented briefing modules

docs/internal/briefing.md

@@ -23,6 +23,8 @@ Outputs:
   object for Daily, 3-Day, Weekend, or Storm Report
 - module registry definitions for known module IDs, stanza names, option
   shapes, fact requirements, report compatibility, and missing-data behavior
+- source-oriented module outputs for `metadata`, `current_conditions`,
+  `alert_digest`, `area_forecast_discussion`, and `weather_story`
 - optional `currentConditions` prompt context from normalized
   `/conditions/current` data when available
 - optional structured `weatherStory` context on report-specific briefing
@@ -73,6 +75,10 @@ None. Builders either return a complete briefing package or an error.
   stanza names.
 - Module composition validation rejects unknown modules, duplicate modules,
   incompatible report/module combinations, and invalid typed options.
+- Source-oriented module builders omit missing optional current conditions,
+  forecast discussion, and weather story stanzas.
+- Alert digest output distinguishes checked empty alert data from missing alert
+  source data.
 - Save failures include path and operation context.
 
 ## Tests
@@ -83,6 +89,7 @@ Inspect:
 - `internal/briefing/three_day_test.go`
 - `internal/briefing/weekend_test.go`
 - `internal/briefing/storm_test.go`
+- `internal/briefing/base_modules_test.go`
 - `internal/briefing/modules_test.go`
 - `internal/app/app_test.go`