feat: Ruleset Management — D&D rulesets and custom rule configuration

[Fortinbra]

Spec for ruleset management. Support standard D&D 5e rules as a baseline plus the ability to define custom rulesets, override individual rules, and create house rules that govern how the AI DM applies mechanics.

[Fortinbra]

✅ Spec approved by The Doctor. Ready for Danny's TDD kickoff ceremony.

Review findings:

• ✓ Base rulesets are read-only (correctly enforced - no PUT/DELETE endpoints)
• ✓ HouseRules singleton pattern is sound
• ✓ Rule text preserved verbatim for Ollama consumption (future)
• ✓ No Human DM override needed (rules are DM-authored, not AI-generated)
• ✓ CQRS pattern applied correctly

Dependencies: None (independent from spec #4)

Agent assignment: Rory (domain entities, API endpoints, migrations), Clara (UI pages for rulesets)