Codify MXC agent and code-review guidance

[Branden Bonaby (bbonaby)]

📖 Description

• Add shared agent expectations for maintainable design, readable documentation, and meaningful functional and regression coverage.
• Add MXC review checks for SDK breaking changes, weakened CI gates, and useful consumer-facing SDK documentation.
• Connect Copilot, Claude, and contributor guidance to the shared rules and review skill.

Documentation and agent instructions only; no runtime or CI workflow changes.

🔗 References

No linked issue.

🔍 Validation

• git diff --check origin/main...user/bbonaby/codify-repo-instructions — passed.
• git rev-list --count origin/main..user/bbonaby/codify-repo-instructions — passed; one commit.
• Runtime and SDK tests not run: documentation and agent instructions only.

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)
• If this PR changes Cargo.lock, the dependency-feed-check check passes (see pull request builds)

📋 Issue Type

• Bug fix
• Feature
• Task

---

GitHub Actions runs the PR validation build automatically. The ADO pipeline
(MXC-PR-Build) is the Azure version of the PR pipeline, kept in parity with the GitHub
Actions build; it runs on merge to main, and Microsoft reviewers with write access can trigger it
on a PR with /azp run. See pull request builds.

If the dependency-feed-check check fails on a new dependency, the crate must be added to
the feed before the PR can pass. See pull request builds
for the steps.

Microsoft Reviewers: Open in CodeFlow

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

🟢 Approval recommended

The documentation-only changes have no identified blocking defects and require no runtime tests.

No open issues 🎉

What changed in this PR

Codifies shared development and review expectations for MXC without changing runtime behavior or CI workflows.

Changes:

• Adds shared design, documentation, and testing guidance.
• Defines review checks for SDK compatibility, CI coverage, and containment.
• Connects contributor, Copilot, and Claude guidance.

File	Description
CONTRIBUTING.md	Clarifies testing expectations and links shared guidance.
CLAUDE.md	Points Claude to shared instructions and review guidance.
AGENTS.md	Defines design, communication, and validation expectations.
.github/​skills/​mxc-code-review/​SKILL.md	Adds repository-specific review checks.
.github/​copilot-instructions.md	Links shared engineering and review guidance.

_{Review effort: Balanced}