Add 3-Day Outlook generation

docs/internal/briefing.md

@@ -6,7 +6,7 @@ This document describes the implemented briefing package boundary.
 
 `internal/briefing` builds structured report-specific briefing packages from
 forecast summaries and report metadata. The package currently implements Daily
-Today and Daily Tomorrow briefing content.
+Today, Daily Tomorrow, and 3-Day Outlook briefing content.
 
 ## Inputs and Outputs
 
@@ -14,12 +14,13 @@ Inputs:
 
 - resolved report definition and valid period
 - forecast bundle
-- derived daily forecast summary
+- derived forecast summary or summaries
 - configured units and timezone
 
 Output:
 
-- `briefing.Package` JSON containing common metadata and Daily briefing content
+- `briefing.Package` JSON containing common metadata and report-specific
+  briefing content
 
 ## Boundaries
 
@@ -37,12 +38,18 @@ Output:
   story context when available.
 - Daily Tomorrow also includes planning inputs for morning readiness,
   commute/school/workday concerns, and what may change overnight.
+- 3-Day content includes one summary per local day or partial day, with overall
+  character, temperature range, precipitation, wind, risk, outdoor-window, and
+  alert inputs, plus broader discussion and weather-story context when
+  available.
 - Briefing JSON is written atomically by `briefing.Save`.
 
 ## Failure Behavior
 
 - Daily briefing construction requires a Daily report definition and a derived
   daily forecast summary.
+- 3-Day briefing construction requires a 3-Day report definition and at least
+  one derived daily summary in the outlook period.
 - Save failures include path and operation context.
 
 ## Tests
@@ -50,6 +57,7 @@ Output:
 Inspect:
 
 - `internal/briefing/daily_test.go`
+- `internal/briefing/three_day_test.go`
 - `internal/app/app_test.go`
 - `internal/cli/root_test.go`