[Enhancement] Add CONTRIBUTING.md as org default

[CTristan]

Summary

Add CONTRIBUTING.md at the repo root, inherited org-wide by repos without their own. Adapt from lobotomy-corporation-mods/CONTRIBUTING.md, generalizing mod-specific wording.

Why

Sets baseline contribution expectations (dev setup, testing, code style, SPDX headers, CSharpier formatting, warnings-as-errors) across every org repo. Child repos can still override with their own file if they need repo-specific guidance.

Acceptance Criteria

• CONTRIBUTING.md exists at repo root
• Covers: how to report issues, how to propose changes, PR process
• References org-wide code quality standards from CLAUDE.md (warnings-as-errors, SPDX headers, CSharpier, analyzer severity)
• Generic enough to apply to both mod repos and tooling repos
• Links to the relevant dotnet ci commands for local verification