feat: persist ConversationContext across sessions

[luceinaltis]

Context

qracer/conversation/context.py holds ConversationContext (current_topic, topic_stack, intent, depth, last_activity) in memory only. It's reconstructed from the JSONL log on each session start but loses cross-session continuity. docs/user-experience.md line 80 marks this as "구현 예정".

Goal

Persist ConversationContext state so returning users get continuity ("last time you were looking at AAPL earnings...").

Scope

• Serialize ConversationContext to JSON on session end / compaction
• Store in FactStore or a dedicated context.json file in ~/.qracer/
• Load on session start, merge with FactStore open theses for a coherent "welcome back" experience
• Decay stale topics (>7 days old → remove from topic_stack)

Related

• FactStore provides thesis continuity (feat: structured fact store for cross-session memory persistence #156)
• SessionDigest (feat: SessionDigest — structured replacement for Tier 2 free-text summaries #158) captures session-level metadata
• Session-start briefing (feat: watchlist-based daily briefing on session start (closes #140) #149) is the natural display point