A16: Convert game modes to TOML #136

Summary

Convert the currently hardcoded game mode definitions (Deathmatch, Team Deathmatch, Faction Deathmatch) to TOML files.

Deliverables

• `modes/deathmatch.toml` -- FFA mode with frag/time limits
• `modes/team-deathmatch.toml` -- Team mode with team scoring
• `modes/faction-deathmatch.toml` -- Faction-based teams

Each mode file defines: name, description, team settings, default limits, and event handler bindings.

Reference

• TOML Reference -- Game mode format

Acceptance Criteria

• Three stock mode TOML files created
• Server loads mode from TOML instead of hardcoded logic
• Game behavior unchanged when using TOML mode files
• Mod can provide a custom mode TOML and it loads correctly

Depends on: #124 (TOML parser)
Blocks: #120