Add Daily prompt input preflight

docs/internal/prompt-input.md

@@ -0,0 +1,52 @@
+# Prompt Input Internals
+
+This document describes the implemented prompt input package boundary.
+
+## Purpose
+
+`internal/promptinput` converts a structured briefing package into the
+`data_package` JSON file passed to `scriptorium` prompts.
+
+## Inputs and Outputs
+
+Input:
+
+- `briefing.Package`
+
+Output:
+
+- `promptinput.Package` JSON with report metadata, briefing content, source
+  warnings, RunID, and an empty Recent Changes section.
+
+## Boundaries
+
+- This package owns the prompt input schema and required-field validation.
+- It does not fetch weather data, compute forecast summaries, compare prior
+  snapshots, or invoke `scriptorium`.
+
+## Behavior
+
+- `promptinput.Build` copies report metadata from the briefing package.
+- `promptinput.Validate` rejects missing or inconsistent required fields before
+  render preflight.
+- `promptinput.Save` writes JSON atomically where practical.
+- Recent Changes is present as an empty `items` list until structured comparison
+  is implemented.
+
+## Failure Behavior
+
+Validation errors name the missing or inconsistent field. Save failures include
+the filesystem operation and path context.
+
+## Tests
+
+Inspect:
+
+- `internal/promptinput/package_test.go`
+- `internal/app/app_test.go`
+
+## Invariants
+
+- Prompt input data remains structured JSON.
+- Briefing metadata and top-level report metadata must agree.
+- Recent Changes is not inferred from rendered report text.