feat: Implement bidirectional Story Bible links with draft text

- Add story_bible.rs module with StoryBibleIndex, StoryBibleItem, and
  StoryBibleReference types for indexing Story Bible content
- Support wiki-style [[...]] and mention-style @... syntax for references
- Highlight Story Bible references in editor with purple background
- Add Story Bible window (View > Tools > Story Bible) to browse items
- Show references from current draft file in Story Bible window
- Allow clicking references to jump to their location in the editor
- Auto-index Story Bible items from common directories (03-Characters,
  02-Story Bible, Characters, etc.)
- Extract metadata from YAML frontmatter or filenames
- Add walkdir dependency for directory scanning
- Fix pre-existing format string errors in workspace.rs

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

src/app/ui.rs

@@ -331,6 +331,10 @@ impl App {
                         ui.close_menu();
                         self.show_characters = true;
                     }
+                    if ui.button("📖 Story Bible…").clicked() {
+                        ui.close_menu();
+                        self.show_story_bible = true;
+                    }
                     ui.separator();
                     let busy = self.beats_rx.is_some();
                     if ui