Add Daily recent changes comparison

docs/internal/changes.md

@@ -0,0 +1,66 @@
+# Changes Internals
+
+This document describes the implemented structured change comparison boundary.
+
+## Purpose
+
+`internal/changes` compares current and prior structured briefing snapshots and
+produces compact change records for prompt input data packages.
+
+## Inputs and Outputs
+
+Inputs:
+
+- prior Daily briefing package
+- current Daily briefing package
+- configured Recent Changes thresholds
+
+Output:
+
+- ordered `changes.Change` records with type, message, previous value, and
+  current value where useful
+
+## Boundaries
+
+- This package compares structured briefing data only.
+- It does not read state directly, render Markdown, invoke `scriptorium`, or
+  compare generated report text.
+
+## Config Fields Used
+
+The app maps these config fields into comparison thresholds:
+
+- `recent_change.temperature_degrees`
+- `recent_change.precip_probability_points`
+- `recent_change.wind_gust_miles_per_hour`
+- `recent_change.precip_timing_shift_minutes`
+
+## Behavior
+
+Daily comparison currently detects:
+
+- temperature changes crossing configured thresholds
+- precipitation probability and timing changes
+- alert additions and removals
+- peak wind gust changes
+- snow, ice, and thunder risk changes
+
+When no prior comparable snapshot exists, the app sends an empty Recent Changes
+section in the data package.
+
+## Failure Behavior
+
+Daily comparison requires both inputs to contain Daily briefing content.
+
+## Tests
+
+Inspect:
+
+- `internal/changes/daily_test.go`
+- `internal/app/app_test.go`
+
+## Invariants
+
+- Recent Changes are based on structured snapshots, not Markdown report text.
+- Comparison thresholds come from configuration.
+- The comparison output remains compact enough for prompt input.

docs/internal/prompt-input.md

@@ -16,7 +16,7 @@ Input:
 Output:
 
 - `promptinput.Package` JSON with report metadata, briefing content, source
-  warnings, RunID, and an empty Recent Changes section.
+  warnings, RunID, and a Recent Changes section.
 
 ## Boundaries
 
@@ -30,8 +30,8 @@ Output:
 - `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.
+- Recent Changes is present as an `items` list. It is empty when no prior
+  comparable snapshot exists or no meaningful changes are detected.
 
 ## Failure Behavior
 
@@ -43,6 +43,7 @@ the filesystem operation and path context.
 Inspect:
 
 - `internal/promptinput/package_test.go`
+- `internal/changes/daily_test.go`
 - `internal/app/app_test.go`
 
 ## Invariants

docs/internal/state.md

@@ -26,6 +26,7 @@ Outputs:
 - Markdown report path
 - metadata JSON
 - prior Daily snapshot metadata when available
+- prior Daily briefing package when loaded by path
 
 ## Boundaries
 
@@ -49,7 +50,8 @@ Workspace subdirectories must be relative paths that stay under
 Managed artifact names use RunID, which is generated from report generation time
 and report ID. Daily metadata is stored beside Daily briefing snapshots by valid
 local date. Prior Daily snapshot lookup reads metadata for the same valid local
-date and returns the latest earlier run. The store prepares the managed Markdown
+date and returns the latest earlier run. The store can load a briefing snapshot
+by path for structured comparison. The store prepares the managed Markdown
 report path before `scriptorium run` writes it.
 
 ## Failure Behavior

docs/operations.md

@@ -63,6 +63,15 @@ Each Daily generation writes metadata that links:
 - preflight output path
 - rendered report path
 
+## Recent Changes
+
+When a prior comparable Daily briefing snapshot exists for the same valid local
+date, the app compares structured briefing data before writing the prompt input
+data package. Meaningful changes are included under `recentChanges.items`.
+
+When no prior comparable snapshot exists, or no configured threshold is crossed,
+the Recent Changes list is empty.
+
 ## Recovery
 
 If render preflight exits nonzero after producing a result, the captured stdout,