fix: exclude __words__ keyword index from corruption detection and getStats()

The __words__ keyword index stores 50-5000 entries per entity (one per
word), which inflated avg entries/entity well above the corruption
threshold of 100. This caused:

1. validateConsistency() to falsely detect corruption on every startup,
   triggering unnecessary clearAllIndexData() + rebuild() cycles
2. getStats() to log false "Metadata index may be corrupted" warnings
   and report inflated totalEntries/totalIds stats

Both methods now skip __words__ when counting, so stats and health
checks reflect metadata fields only (noun, type, createdAt, etc.).
Keyword search is unaffected since the __words__ field index itself
is not modified.

src/cli/commands/utility.ts

@@ -133,7 +133,7 @@ export const utilityCommands = {
       // removeOrphans and rebuildIndex would require new Brainy APIs
       if (options.removeOrphans || options.rebuildIndex) {
         spinner.warn('Advanced cleanup options not yet implemented')
-        console.log(chalk.yellow('\n⚠️  Advanced cleanup features coming in v3.21.0:'))
+        console.log(chalk.yellow('\n⚠️  Advanced cleanup features coming soon:'))
         console.log(chalk.dim('  • --remove-orphans: Remove disconnected items'))
         console.log(chalk.dim('  • --rebuild-index: Rebuild vector index'))
         console.log(chalk.dim('\nUse "brainy clean" without options to clear the database'))