Add Tomorrow generated text contract

internal/reporttemplate/prompts/tomorrow.generated_text.md

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+# Tomorrow Generated Text
+
+Create structured prose for the Tomorrow Report using the supplied
+weatherreporter data package.
+
+Return JSON that matches the Scriptorium-registered schema for
+`weather.tomorrow_generated_text`.
+
+Fields:
+
+- `summary`: concise overview of tomorrow's weather.
+- `forecast_discussion`: one or more paragraph strings explaining the main
+  weather drivers and forecast reasoning.
+- `precipitation_timing`: optional plain-language precipitation timing context.
+- `confidence`: optional uncertainty or confidence note.
+
+Use deterministic facts from the data package for weather details. Do not
+invent watches, warnings, precipitation windows, temperatures, or timing.