chore: add FunPlay Unity MCP setup

[JOY (JOY)]

Summary

• Add FunPlay Unity MCP package pin at v0.3.3 commit b600f4e.
• Add project-local Codex/FunPlay MCP workflow guidance generated from the Unity package, cleaned for repo markdown style.
• Add Unity project AGENTS.md for FunPlay-specific MCP operating rules.

Local review

• Local code-review fallback: APPROVED WITH SUGGESTIONS.
• No blockers found. This only adds tooling configuration and package metadata.
• Suggestion: keep Coplay and FunPlay both available until FunPlay has enough local smoke time to become the default bridge.

Tests

• dotnet build Unity/Assembly-CSharp-Editor.csproj --nologo (third-party Photon/Fusion warnings only, 0 errors)
• npx --yes markdownlint-cli2@0.13.0 Unity/AGENTS.md Unity/.codex/skills/funplay-unity-mcp-workflow/SKILL.md
• git diff --cached --check before commit
• Em dash scan for staged FunPlay files
• Local FunPlay MCP tools/list smoke at http://127.0.0.1:8765/mcp returned tools successfully

Codex (@codex) please review this PR.