Phase 0: implement shared runtime, world-state, and command contracts

[vultrdev]

Objective

Create the first real contracts-in-code package for Agentarium.

Scope

• define runtime event types
• define world state types
• define command types
• keep contracts separate from UI concerns
• align with the docs in agentarium-docs

References

• agentarium-docs/docs/contracts/runtime-events.md
• agentarium-docs/docs/contracts/world-state.md
• agentarium-docs/docs/contracts/commands.md
• agentarium-docs/docs/implementation/phase-0-build-plan.md

Done when

• package exports typed contracts
• naming matches the public docs closely
• the app can import the contracts cleanly
• changelog and docs are updated if semantics shifted