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>

Cargo.toml

@@ -12,6 +12,7 @@ serde_json = "1"
 pulldown-cmark = { version = "0.12", default-features = false }
 zip = { version = "2", default-features = false, features = ["deflate"] }
 dirs = "5"
+walkdir = "2"
 # Native open/save dialogs via the XDG Desktop Portal (no GTK build dependency).
 rfd = { version = "0.15", default-features = false, features = ["xdg-portal", "async-std"] }
 # Blocking HTTP client for talking to a LanguageTool server. The rustls TLS