feat: remove legacy ImportManager, standardize getStats() API

- Removed ImportManager class and exports (use brain.import() instead)
- Fixed all documentation: getStatistics() → getStats()
- Updated 41 files across codebase for consistency
- Removed ImportManager section from API docs
- Added v3.30.0 migration guide to CHANGELOG

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

bin/brainy-interactive.js

@@ -444,7 +444,7 @@ async function showStatistics(brain) {
   const spinner = ora('Gathering statistics...').start()
   
   try {
-    const stats = await brain.getStatistics()
+    const stats = brain.getStats()
     spinner.succeed('Statistics loaded')
     
     console.log(boxen(