Add Tomorrow planning brief generation

docs/internal/briefing.md

@@ -5,8 +5,8 @@ This document describes the implemented briefing package boundary.
 ## Purpose
 
 `internal/briefing` builds structured report-specific briefing packages from
-forecast summaries and report metadata. The package currently implements the
-Daily briefing only.
+forecast summaries and report metadata. The package currently implements Daily
+Today and Daily Tomorrow briefing content.
 
 ## Inputs and Outputs
 
@@ -35,6 +35,8 @@ Output:
 - Daily content includes bottom-line inputs, daypart summaries, relevant alerts,
   outdoor window inputs, narrative periods, discussion context, and weather
   story context when available.
+- Daily Tomorrow also includes planning inputs for morning readiness,
+  commute/school/workday concerns, and what may change overnight.
 - Briefing JSON is written atomically by `briefing.Save`.
 
 ## Failure Behavior