feat: persistent self-learning memory + curiosity engine

- New memory.js: JSON-backed MemoryStore with 5 categories (lesson, pattern, preference, discovery, gotcha)
- Memory injected into system prompt — bot sees past learnings every session
- Curiosity engine: auto-detects errors/fixes, corrections, successful patterns, new tool discoveries
- New commands: /memory (stats), /remember (save), /recall (search), /forget (delete)
- Runs AFTER response delivery — zero latency impact
- 500 memory cap with smart eviction (keeps gotchas/lessons, evicts old discoveries)
- data/ directory gitignored (memory is local to each deployment)

.gitignore

@@ -2,5 +2,6 @@ node_modules/
 .env
 .zcode.config.json
 logs/
+data/
 *.log
 .DS_Store