Add ARCHITECTURE.md with harness boundaries and component overview

[northline-lab]

Summary

CONTRIBUTING.md directs maintainers and contributors to boundaries in ARCHITECTURE.md, but that file is missing. This creates confusion and breaks documentation expectations. Provide a minimal, clear ARCHITECTURE.md summarizing the boundaries and components.

Verification

• aci_suggest_verification proposed pytest and compileall.
• python3 -m pytest could not run due to missing deps in this minimal environment; no code change required.
• python3 -m compileall . completed successfully, indicating the doc addition doesn’t affect code.
• Patch submission initially flagged due to transient pycache files created by compileall; removed them and resubmitted.
• Final patch adds ARCHITECTURE.md only.

Risk

• Selected task risk: low
• Files changed: ARCHITECTURE.md

Dry-Run Notice

This is a dry-run PR draft. No external repository write was performed.