fix: Correct database API usage in stats command - achieve 100% system health

Fixed the database connection error that was causing degraded system health by
using the proper DatabaseManager API instead of non-existent methods.

## Root Cause Analysis:
- **Issue**: `_show_core_system_stats()` tried to call `db_manager.get_connection()`
- **Problem**: DatabaseManager class doesn't have a `get_connection()` method
- **Impact**: System health reported as "Degraded (66.7%)" due to database unavailability

## Why No Tests Caught This:
1. **Existing tests** only test public API methods (`store_markdown_file`, `get_markdown_file`, etc.)
2. **No tests existed** for `get_connection()` because the method doesn't exist
3. **New stats function** was the first code to assume this method existed
4. **Database pattern**: Uses temporary connections per operation, not persistent connections

## Solution Applied:
- **Replaced** `conn = db_manager.get_connection()` with proper `execute_query()` API
- **Updated queries** to use named columns: `SELECT COUNT(*) as count FROM table`
- **Added resilience** for optional tables (schema_files) with try/catch
- **Result**: System health now reports ✅ **100% Healthy**

## Changes Made:
```python
# Before (broken):
conn = db_manager.get_connection()
cursor.execute("SELECT COUNT(*) FROM markdown_files")
total_files = cursor.fetchone()[0]

# After (correct):
result = db_manager.execute_query("SELECT COUNT(*) as count FROM markdown_files")
total_files = result[0]['count'] if result else 0
```

## Current System Health:
```
🏥 System Health: ✅ Healthy (100.0%)
   Healthy Subsystems: 3/3

🗄️  Database: ✅ Available (56.0 KB) - 11 files processed
🗃️  Cache: ✅ Available (0 B)
🖥️  AST Service: ✅ Available
```

This fix demonstrates the value of the health monitoring system - it successfully
identified a real integration issue and provided clear diagnostic information.

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

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

markitect/cli.py

@@ -288,25 +288,26 @@ def _show_core_system_stats(config, format):
                     db_size = 0
                     db_size_human = '0 B'
 
-                # Get file counts from database
+                # Get file counts from database using proper API
                 try:
-                    conn = db_manager.get_connection()
-                    cursor = conn.cursor()
-
-                    # Total files
-                    cursor.execute("SELECT COUNT(*) FROM markdown_files")
-                    total_files = cursor.fetchone()[0]
+                    # Use execute_query method for database queries
+                    total_files_result = db_manager.execute_query("SELECT COUNT(*) as count FROM markdown_files")
+                    total_files = total_files_result[0]['count'] if total_files_result else 0
 
                     # Recent files (last 7 days)
-                    cursor.execute("""
-                        SELECT COUNT(*) FROM markdown_files
+                    recent_files_result = db_manager.execute_query("""
+                        SELECT COUNT(*) as count FROM markdown_files
                         WHERE created_at >= datetime('now', '-7 days')
                     """)
-                    recent_files = cursor.fetchone()[0]
+                    recent_files = recent_files_result[0]['count'] if recent_files_result else 0
 
-                    # Schema count
-                    cursor.execute("SELECT COUNT(*) FROM schema_files")
-                    schema_count = cursor.fetchone()[0]
+                    # Schema count (table might not exist)
+                    try:
+                        schema_count_result = db_manager.execute_query("SELECT COUNT(*) as count FROM schema_files")
+                        schema_count = schema_count_result[0]['count'] if schema_count_result else 0
+                    except Exception:
+                        # schema_files table doesn't exist - this is okay
+                        schema_count = 0
 
                     stats['database'] = {
                         'path': db_path,