feat: add model-backed NPC society dialog

[JOY (JOY)]

Summary

• Add Nakama-owned NPC society/dialog orchestration with prompt traces, knowledge packs, conversation sessions, and separate autonomous vs direct-chat DOS.AI budgets.
• Add Unity prototype NPC chat/dialog UI, direct chat priority, input lock, dialog facing, model/fallback status visibility, and local request scheduling.
• Document NPC society architecture and update roadmap/changelog.

Local review

• Local code-review fallback: APPROVED WITH SUGGESTIONS.
• No blockers found in the focused diff.
• Specialist Task review is unavailable in this Codex session, so this used the repo code-review checklist manually.
• Main suggestion: move prototype polling and scene scans into the Fusion/dedicated-server lane before production.

Tests

• npm run build in backend/nakama
• npm test in backend/nakama
• dotnet build Unity/SecondSpawn.Networking.csproj --nologo
• dotnet build Unity/SecondSpawn.AI.csproj --nologo
• dotnet build Unity/SecondSpawn.UI.csproj --nologo
• npx --yes markdownlint-cli2@0.13.0 README.md ROADMAP.md CHANGELOG.md CONTRIBUTING.md SECURITY.md AGENTS.md .claude/CLAUDE.md "docs/**/*.md"
• git diff --check
• Unity FunPlay recompile: no errors
• Unity Play Mode smoke: 10 permanent NPCs plus local player spawned, no new console errors
• Live local Nakama DOS.AI smoke: secondspawn_agent_decide returned source=model and source_reason=dos_ai_validated_intent

Codex (@codex) please review this PR.

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