docs: add GameHours engineering constitution

[Ayerdi]

Summary

Introduce the lightweight SDD governance structure discussed for GameHours:

• add docs/CONSTITUTION.md with the durable, non-negotiable engineering principles;
• simplify root AGENTS.md so it remains a small operational context file rather than duplicating roadmap, findings and project philosophy.

Why

The previous AGENTS.md mixed permanent engineering principles, time-sensitive findings, Codex-specific subagent policy and operational guidance. That increases context cost and makes the file age quickly.

The new split follows a lightweight Spec-Driven approach:

• AGENTS.md: project summary, architecture map, commands, essential boundaries and finish checks;
• docs/CONSTITUTION.md: durable principles that every spec/plan/implementation/review must respect;
• roadmap/spec/plan documents keep feature-specific direction and decisions.

Constitution principles

1. research before deciding;
2. simplicity and reuse first;
3. evidence and root cause over assumptions;
4. preserve data truth and provenance;
5. local-first and optional integrations;
6. measure performance before optimizing;
7. product quality includes UX;
8. validate before claiming completion.

It also records the intended lightweight SDD loop for substantial work without requiring heavyweight specs for obvious small fixes.

Scope

Documentation/governance only. No production code, dependencies, CI configuration or runtime behavior changed.

Validation

PR CI/CodeQL will validate the repository after this documentation-only change.