feat: persist NPC conversation turns

[JOY (JOY)]

Summary

• Add secondspawn_npc_conversation_get so clients and debug tools can inspect an active NPC conversation session.
• Persist compact recent_turns on conversation sessions, including the triggering player line and each NPC reply.
• Let follow-up player chat events reuse an existing conversation_session_id instead of creating a fresh stateless session every message.
• Include recent_turns in the DOS.AI decision prompt conversation context.
• Update Unity chat routing to remember the active session and pass it on follow-up chat messages.

Why

NPC chat was model-backed and session-aware, but follow-up player messages could still create fresh sessions. This makes a 1:1 chat behave more like an RPG conversation: the NPC can see what was just said and clients can inspect the session state.

Safety

• The model still receives read-only context only.
• LLM output still goes through the existing validated intent path.
• Unity sends only session identifiers and player text. No provider keys or model credentials are exposed.

Local Code Review Fallback

Verdict: APPROVED.

• Hard Rule feat(backend): add Nakama OSS local base #2 remains satisfied: model output cannot mutate authoritative game state.
• Hard Rule feat(controller): add Simple KCC controller base #3 remains satisfied: Unity does not receive model API keys.
• No ADR references were found in touched files, so review used AGENTS.md and existing Nakama/Fusion/api.dos.ai boundaries.
• .Codex/docs/technical-preferences.md is not present, so specialist configuration was skipped.

TDD Evidence

• RED: test expected 26 registered RPCs plus session recent_turns and failed because secondspawn_npc_conversation_get did not exist.
• GREEN: new tests pass after adding the RPC, session turn persistence, follow-up session reuse, and prompt context injection.

Verification

• npm.cmd install in backend/nakama
• npm.cmd run build in backend/nakama
• npm.cmd test in backend/nakama
• git diff --check --cached
• Staged-file em-dash scan

Verification Note

Unity C# compile still needs Editor/root-checkout verification because isolated worktrees do not contain Unity Library package assemblies. The Unity changes are DTO string fields plus session-id routing in the existing chat script.

Unrelated Files Left Out

The generated knowledge-pack file was touched by npm run build line endings only and was intentionally not staged.