docs: track conversation memory progress

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Summary

• update ROADMAP with closed conversation memory and player relationship ledger progress
• update CHANGELOG with the merged Nakama conversation memory changes

Local review

• Local code-review fallback: APPROVED for docs-only tracking update
• No ADR drift: documents the already-merged Nakama backend behavior

Tests

• npx markdownlint-cli ROADMAP.md CHANGELOG.md
• git diff --check
• em-dash scan on changed diff

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