feat: carry NPC conversation session context

[JOY (JOY)]

Summary

• Carry conversation_session_id and trigger_event_id from Unity NPC decision requests into Nakama.
• Inject the active conversation session into the DOS.AI decision prompt, including objective, participants, current turn count, turns remaining, and compact transcript summary.
• Seed new conversation sessions with the triggering player line so the first NPC reply and later follow-ups have shared context.
• Add a regression test proving a later player question sees both the original player line and the previous NPC answer in the model prompt.

Why

NPC chat was model-backed, but each model decision could still feel stateless. This patch makes the current Nakama conversation session part of the prompt, so follow-up questions can reference what was just said instead of sounding like a fresh call.

Safety

• LLM output still goes through the existing validated intent path.
• Session context is read-only prompt context. It does not let the model mutate authoritative game state.
• Unity client sends only session identifiers and trigger identifiers, not provider keys or model credentials.

Local Code Review Fallback

Verdict: APPROVED.

• Hard Rule feat(backend): add Nakama OSS local base #2 remains satisfied: model output still cannot mutate authoritative game state.
• Hard Rule feat(controller): add Simple KCC controller base #3 remains satisfied: Unity never receives or sends provider API keys.
• No ADR references were found in the touched files, so ADR compliance was checked against AGENTS.md and existing Nakama/Fusion/api.dos.ai boundaries.

Verification

• TDD red: new prompt-history assertion failed because conversation_session was missing from the DOS.AI prompt.
• npm.cmd run build in backend/nakama
• npm.cmd test in backend/nakama
• git diff --check --cached
• Staged-file em-dash scan

Verification Note

dotnet build Unity\SecondSpawn.AI.csproj could not be completed inside the isolated worktree because Unity-generated project files and Library package assemblies are ignored and not present there. Copying .csproj files locally still left Unity package references such as Unity.InputSystem unresolved. The C# change is limited to adding DTO string fields and assigning existing string values; Unity compile should be checked from the main Unity checkout or Editor after merge.

Unrelated Files Left Out

The worktree has a generated knowledge-pack line-ending touch from npm run build; it is intentionally not staged.