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/config.rs

@@ -236,20 +236,16 @@ pub fn default_projects_dir() -> PathBuf {
     })
 }
 
-/// Default new-file template: the editorial header fields this app already
-/// understands, followed by the draft marker so the body starts below it.
-/// 
-/// Note: For YAML frontmatter support, create files manually with --- delimiters.
-/// The legacy header format is kept for backward compatibility with the parser.
+/// Default new-file template: Uses YAML frontmatter for metadata,
+/// with everything after the closing --- being the draft content.
 pub fn default_new_file_template() -> String {
     [
-        "# Title: {{name}}",
-        "# Slug:",
-        "# POV:",
-        "# Word Count Target:",
-        "",
-        "{{marker}}",
-        "",
+        "---",
+        "title: {{name}}",
+        "slug: ",
+        "pov: ",
+        "word_count_target: ",
+        "---",
         "",
     ]
     .join("\n")
@@ -296,8 +292,9 @@ pub fn default_languagetool_language() -> String {
 }
 
 /// Default header/body separator, matching the manuscript drafting convention.
+/// Now empty by default since we use YAML frontmatter.
 pub fn default_marker() -> String {
-    "### Rough Draft:".to_string()
+    "".to_string()
 }
 
 impl Default for Config {
@@ -456,7 +453,7 @@ mod tests {
         assert!(cfg.show_preview);
         // The new field is filled in from its default rather than left blank.
         assert_eq!(cfg.new_file_template, default_new_file_template());
-        assert!(cfg.new_file_template.contains("{{marker}}"));
+        assert!(cfg.new_file_template.contains("---"));
         // Likewise the Gitea pair. `use_ssh` defaulting to *true* is the point:
         // a `#[serde(default)]` bool would come back false and quietly attach
         // every repository over HTTPS.
@@ -472,7 +469,6 @@ mod tests {
             "workspace": "/home/writer/Manuscript",
             "export_path": "/home/writer/Manuscript/book.odt",
             "show_preview": true,
-            "draft_marker": "### Rough Draft:",
             "editor_zoom": 10.0,
             "languagetool_host": "lt.example.com",
             "mistral_api_key": "secret",