Add Daily briefing JSON generation

docs/internal/briefing.md

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+# Briefing Internals
+
+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.
+
+## Inputs and Outputs
+
+Inputs:
+
+- resolved report definition and valid period
+- forecast bundle
+- derived daily forecast summary
+- configured units and timezone
+
+Output:
+
+- `briefing.Package` JSON containing common metadata and Daily briefing content
+
+## Boundaries
+
+- Briefings are structured weather facts and context for later prompt input.
+- This package does not fetch weather data, compare prior snapshots, build
+  `scriptorium` data packages, or render final report prose.
+
+## Behavior
+
+- Common metadata includes schema version, RunID, report ID, variant, prompt ID,
+  generation time, units, timezone, valid period, source location, source
+  provenance, hashes, and source warnings.
+- Daily content includes bottom-line inputs, daypart summaries, relevant alerts,
+  outdoor window inputs, narrative periods, discussion context, 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.
+- Save failures include path and operation context.
+
+## Tests
+
+Inspect:
+
+- `internal/briefing/daily_test.go`
+- `internal/app/app_test.go`
+- `internal/cli/root_test.go`
+
+## Invariants
+
+- Weather facts come from normalized and derived source data.
+- Briefing output remains JSON-inspectable.
+- LLM prompt input packaging and `scriptorium` execution remain outside this
+  boundary.