feat: persist closed NPC conversation memory

[JOY (JOY)]

Summary

• persist a durable relationship memory summary when an NPC conversation session closes
• mark closed conversation objectives as complete even when the previous stored objective was active
• preserve existing runtime memories while writing the closed-conversation summary

Local review

• Local code-review fallback: APPROVED
• ADR compliance: compliant with Nakama as game backend and server-side LLM intent validation boundary
• Security: no Unity client key path added, no LLM authority expansion, no gameplay state mutation from model output

Tests

• npm run build
• npm test
• git diff --check
• em-dash scan on changed diff

[JOY (JOY)]

Codex (@codex) review

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