feat: Remove fallback to Cookiecutters, use bundled template only

- Remove fallback to ~/Documents/Cookiecutters/snowflake from default_project_template()
- Panic with clear error if bundled template not found
- Update tests to use 05-First Draft instead of 06-First Draft
- Update comments to reflect merged structure (05-First Draft contains planning+draft)

The code now only uses the bundled template at templates/snowflake/ relative to the
executable, ensuring all new projects use the merged 05-First Draft structure.

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

src/config.rs

@@ -185,9 +185,9 @@ pub fn default_show_toolbar() -> bool {
 }
 
 /// Default cookiecutter template for **File ▸ New project…**.
-/// First tries the bundled template, then falls back to the user's Cookiecutters directory.
+/// Uses the bundled template from the project's templates/snowflake directory.
 pub fn default_project_template() -> PathBuf {
-    // Try the bundled template first (relative to the executable)
+    // Find the bundled template relative to the executable's location
     if let Ok(exe_path) = std::env::current_exe() {
         // The templates are at the project root level, same as Cargo.toml
         // binary is typically at target/release/md-manuscript or target/debug/md-manuscript
@@ -209,17 +209,14 @@ pub fn default_project_template() -> PathBuf {
             }
         }
     }
-    // Fallback to user's Cookiecutters directory
-    dirs::home_dir()
-        .unwrap_or_else(|| PathBuf::from("."))
-        .join("Documents")
-        .join("Cookiecutters")
-        .join("snowflake")
+    // If we can't find the bundled template, panic with a clear error
+    // The bundled template should always be present in a properly built release
+    panic!("Bundled template not found. Please ensure templates/snowflake/ exists relative to the executable.");
 }
 
 /// Default subfolder opened after generating a project: the snowflake
-/// template's drafting folder, where the chapter files live.
-/// Supports both "05-First Draft" (merged planning+draft) and "06-First Draft" (legacy).
+/// template's drafting folder (05-First Draft), where the chapter files live.
+/// Contains merged planning and draft content in a single folder.
 pub fn default_project_open_subdir() -> String {
     "05-First Draft".to_string()
 }
@@ -594,7 +591,7 @@ mod tests {
     #[test]
     fn bundled_template_is_found_via_exe_path() {
         let path = default_project_template();
-        // The function should find either the bundled template or the fallback
+        // The function should find the bundled template
         // Verify it returns a valid path that contains cookiecutter.json
         assert!(path.join("cookiecutter.json").is_file(),
             "Template path {} should contain cookiecutter.json", path.display());