✨ feat(skill): add user-configurable diagram preferences system

Add a preference system that lets users configure default font, roughness,
fontSize, and strokeWidth — with three scopes (session/folder/global).

Skill layer (Step 1 in SKILL.md):
- Reads .claude/excalidraw-preferences.json (folder) then
  ~/.claude/skills/excalidraw-skill/preferences.json (global)
- If neither exists, prompts user interactively on first use
- Session-only scope keeps preferences in-memory without saving

Server layer (index.ts):
- loadPreferences() reads the same files at startup
- Replaces hardcoded fontFamily ?? 1 with USER_PREFS.fontFamily
- Folder-level preferences override global; user values override both

Also ships preferences.example.json as a template (preferences.json
is gitignored).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -15,6 +15,9 @@ public/dist/
 .cursor/
 .claude/
 
+# User preferences (only the example ships)
+skills/excalidraw-skill/preferences.json
+
 # Development artifacts
 *.excalidraw