feat: redesign memory system — two-layer architecture with grep-based retrieval

nanobot/skills/memory/SKILL.md

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+---
+name: memory
+description: Two-layer memory system with grep-based recall.
+always: true
+---
+
+# Memory
+
+## Structure
+
+- `memory/MEMORY.md` — Long-term facts (preferences, project context, relationships). Always loaded into your context.
+- `memory/HISTORY.md` — Append-only event log. NOT loaded into context. Search it with grep.
+
+## Search Past Events
+
+```bash
+grep -i "keyword" memory/HISTORY.md
+```
+
+Use the `exec` tool to run grep. Combine patterns: `grep -iE "meeting|deadline" memory/HISTORY.md`
+
+## When to Update MEMORY.md
+
+Write important facts immediately using `edit_file` or `write_file`:
+- User preferences ("I prefer dark mode")
+- Project context ("The API uses OAuth2")
+- Relationships ("Alice is the project lead")
+
+## Auto-consolidation
+
+Old conversations are automatically summarized and appended to HISTORY.md when the session grows large. Long-term facts are extracted to MEMORY.md. You don't need to manage this.