fix: exclude assets.db from version control

Remove assets.db from git tracking and add to .gitignore:

**Changes:**
- Remove assets/assets.db from git tracking (was incorrectly committed)
- Add assets/assets.db and **/assets.db patterns to .gitignore

**Rationale:**
- assets.db is a runtime SQLite database containing local asset metadata and usage stats
- Should not be in version control as it contains user-specific operational data
- Similar to markitect.db which was already properly ignored
- Prevents unnecessary binary file commits and merge conflicts

**Database Purpose:**
- Assets system database for tracking file metadata, usage statistics, and processing logs
- Generated and updated automatically during asset management operations
- Project-specific (per-repository) unlike markitect.db (global user database)

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.gitignore

@@ -78,6 +78,8 @@ Thumbs.db
 
 # MarkiTect database files (local development)
 markitect.db
+assets/assets.db
+**/assets.db
 .markitect/
 
 # Issue workspace (temporary development files)