Add AGENTS.md with repository-wide engineering, architecture, and DevSecOps standards

[Copilot]

This PR adds a root-level AGENTS.md that codifies contributor/agent operating standards for this repository. It establishes explicit guardrails across software engineering, architecture, security, and delivery practices, with PowerShell/Pester/Microsoft/GitHub/TDD alignment.

• Scope and intent

  • Introduces /AGENTS.md as the canonical standards document for human and automated contributors.
  • Defines mandatory expectations for safe-by-default change behavior in a system-modifying PowerShell project.

• Engineering + architecture standards

  • Requires small, focused changes and idempotent-by-default behavior.
  • Enforces modular composition (Netclean.psm1-centric), separation of concerns, and testable function boundaries.
  • Adds explicit “Definition of Done” criteria centered on maintainability and correctness.

• Security + DevSecOps standards

  • Formalizes least-privilege/elevation rules, secret-handling constraints, and destructive-action gating.
  • Preserves AV/EDR and virtual adapter protection posture as a non-negotiable baseline.
  • Positions CI/review findings as merge gates and calls out supply-chain hygiene expectations.

• PowerShell, Pester, GitHub, and TDD guidance

  • Aligns with PSScriptAnalyzer policy and PowerShell reliability practices (SupportsShouldProcess, explicit failure handling).
  • Establishes TDD flow for behavior changes and regression expectations for fixes.
  • Defines PR content requirements for risk-aware review.

Example excerpt from the added standards doc:

## Testing Standards (Pester + TDD)
- Apply TDD for new behavior and bug fixes:
  1. Write or update a failing Pester test.
  2. Implement the minimal code change.
  3. Refactor while keeping tests green.
- Add regression tests for every fixed defect.