Have the beat prompt read all proposal attributes

Broaden the plot-beat system prompt beyond characters + plot: instruct
the model to read and honour every attribute the proposal states —
title, setting, genre, tone, characters, target age (adult / YA / middle
grade / …), and the loose plot summary. The beats now follow the stated
genre's conventions, keep the tone, and stay appropriate to the target
age; missing attributes are inferred rather than contradicted. Updates
the user-message framing and the README to match.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XDfaEsTgcn61n3wgP62DFF

src/mistral.rs

@@ -23,9 +23,19 @@ pub const DEFAULT_MODEL: &str = "mistral-large-latest";
 /// numbered beat list per act) so the result drops straight into a manuscript.
 const SYSTEM_PROMPT: &str = "\
 You are a developmental fiction editor who structures novels using the classic \
-three-act structure. You will be given a novel proposal — its characters and a \
-loose plot summary — and must lay the story out as a sequence of concrete plot \
-beats.
+three-act structure. You will be given a novel proposal and must lay the story \
+out as a sequence of concrete plot beats.
+
+First, read the whole proposal and note every attribute it provides. These may \
+include, but are not limited to: the title, setting, genre, tone, characters, \
+target age (e.g. adult, new adult, young adult, middle grade), and a loose plot \
+summary. Honour all of them:
+- Follow the conventions and expected structure of the stated genre.
+- Keep the emotional register consistent with the stated tone.
+- Make the content, stakes, and complexity appropriate to the stated target age.
+- Ground the setting, and use the named characters, throughout the beats.
+If an attribute is absent, infer a sensible choice from the rest of the proposal \
+rather than contradicting what is given.
 
 Output requirements:
 - Return GitHub-flavoured Markdown only. No preamble, commentary, or closing \
@@ -102,8 +112,9 @@ pub fn generate_beats(
     // Frame the proposal so the model reads it as source material, not as new
     // instructions.
     let user = format!(
-        "Here is the novel proposal (characters and a loose plot summary). \
-         Produce the plot beats.\n\n---\n\n{}",
+        "Here is the novel proposal. It may state the title, setting, genre, \
+         tone, characters, target age, and a loose plot summary. Read all of it \
+         and produce the plot beats.\n\n---\n\n{}",
         proposal.trim()
     );
     let request = ChatRequest {