docs: ROADMAP full restore + #98-143 tech debt links + archive historical DESIGN/CHANGES (#144)

docs/archive/DESIGN-0.18.0.md

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+# DESIGN-0.18.0: Memory System
+
+**Version:** 0.18.0  
+**Status:** Phase 1 Complete  
+**Depends on:** compaction (v0.15.0), knowledge bases (v0.14.0), persona-KB binding (v0.17.0), SQLite backend (v0.17.1)
+
+---
+
+## 1. Overview
+
+Memory provides long-term fact persistence across conversations. Unlike
+compaction (within-conversation context) or knowledge bases (static
+documents), memory captures preferences, facts, and context that
+accumulate over time from natural conversation.
+
+**Key differentiator: Persona-scoped memory.** Each Persona builds
+its own memory independently, preventing cross-context contamination.
+A helpdesk Persona accumulates FAQ knowledge; a tutoring Persona
+tracks per-student progress; a user's personal preferences stay
+separate from any Persona context.
+
+---
+
+## 2. Scope Model
+
+Three memory scopes with clear ownership:
+
+```
+┌─────────────────────────────────────────────────┐
+│ user scope                                      │
+│ owner_id = user_id                              │
+│ "Jeff prefers Go over Python"                   │
+│ "Deployment uses Kubernetes + Traefik"          │
+│ Visible in ALL conversations for this user      │
+└─────────────────────────────────────────────────┘
+
+┌─────────────────────────────────────────────────┐
+│ persona scope                                   │
+│ owner_id = persona_id                           │
+│ "Common question: how to reset password"        │
+│ "Policy: refunds within 30 days only"           │
+│ Shared across ALL users of this Persona         │
+└─────────────────────────────────────────────────┘
+
+┌─────────────────────────────────────────────────┐
+│ persona_user scope                              │
+│ owner_id = persona_id, user_id = user_id        │
+│ "Student struggles with recursion"              │
+│ "Customer prefers email over phone"             │
+│ Per-user within a specific Persona              │
+└─────────────────────────────────────────────────┘
+```
+
+**Scope resolution at recall time:**
+
+When a Persona is active, all three scopes are queried and merged
+with priority ordering: persona_user > persona > user. When no
+Persona is active, only user scope is queried.
+
+---
+
+[... full content pasted ...]