feat: Remove ### Rough Draft: marker, use YAML frontmatter only for draft files

- Everything after YAML frontmatter closing --- is now considered the draft content
- No ### Rough Draft: marker needed
- Chapter titles are handled by the program on export based on file order
- Draft content should not include chapter titles
- Updated default_new_file_template to use YAML frontmatter
- Updated preprocess.rs to parse YAML frontmatter for metadata
- Updated all relevant tests
- Updated documentation in help.rs
- Updated template files

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/app/workspace.rs

@@ -813,10 +813,6 @@ order: {}
 
 ---
 
-## Draft Content
-
-### Rough Draft:
-
 [Write your scene content here...]
 
 ---
@@ -2027,14 +2023,14 @@ mod tests {
     #[test]
     fn render_template_expands_every_placeholder() {
         let out = render_template(
-            "# Title: {{name}}\n# Started: {{date}}\n\n{{marker}}\n",
+            "---\ntitle: {{name}}\n---\n",
             "untitled-7",
-            "### Rough Draft:",
+            "",
             "2026-08-22",
         );
         assert_eq!(
             out,
-            "# Title: untitled-7\n# Started: 2026-08-22\n\n### Rough Draft:\n"
+            "---\ntitle: untitled-7\n---\n"
         );
     }
 
@@ -2074,7 +2070,7 @@ mod tests {
         assert_eq!(h.slug, None);
         assert_eq!(h.pov, None);
         assert_eq!(h.goal, None);
-        // Nothing above the marker leaks into the exported prose.
+        // Nothing above the --- leaks into the exported prose.
         assert_eq!(h.body.trim(), "");
     }
 
@@ -2083,8 +2079,7 @@ mod tests {
     fn filled_template_parses_into_header_fields() {
         let marker = crate::config::default_marker();
         let seeded = render_template(
-            "# Title: {{name}}\n# Slug: the door\n# POV: Ada\n\
-             # Word Count Target: 1500 - 2000\n\n{{marker}}\n\nReal prose here.\n",
+            "---\ntitle: {{name}}\nslug: the door\npov: Ada\nword_count_target: 1500 - 2000\n---\n\nReal prose here.\n",
             "chapter-02",
             &marker,
             "2026-08-22",
@@ -2102,11 +2097,12 @@ mod tests {
         let out = render_template(
             &crate::config::default_new_file_template(),
             "chapter-01",
-            "### Rough Draft:",
+            "",
             "2026-08-22",
         );
-        assert!(out.starts_with("# Title: chapter-01\n"), "got {out:?}");
-        assert!(out.contains("\n### Rough Draft:\n"), "got {out:?}");
+        assert!(out.starts_with("---\n"), "got {out:?}");
+        assert!(out.contains("title: chapter-01"), "got {out:?}");
+        assert!(out.contains("---\n"), "got {out:?}");
         assert!(!out.contains("{{"), "placeholder left unexpanded in {out:?}");
     }