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/interactive.ts

@@ -625,7 +625,7 @@ export async function promptCommand(): Promise<string> {
  */
 export async function startInteractiveMode() {
   console.log(chalk.cyan('\n🧠 Brainy Interactive Mode\n'))
-  console.log(chalk.yellow('Interactive REPL mode coming in v3.20.0\n'))
+  console.log(chalk.yellow('Interactive REPL mode coming soon\n'))
   console.log(chalk.dim('Use specific commands for now: brainy add, brainy search, etc.'))
   process.exit(0)
 }