docs: add architecture diagram for scanner pipeline

[mldangelo]

Description

Add a visual architecture diagram showing how ModelAudit's scanner pipeline works. This helps new contributors understand the codebase quickly.

What to include

• CLI entry point → core.scan_model_directory_or_file()
• Scanner registry and priority-based selection
• Detection pipeline (detectors + analysis engines)
• Output formatting (text/JSON/SARIF)

Suggested approach

Create a Mermaid diagram in docs/agents/architecture.md (a section already exists describing the architecture in text). A simple flowchart showing the main components and data flow would be ideal.

References

• modelaudit/cli.py — CLI entry point
• modelaudit/core.py — Scanning orchestration
• modelaudit/scanners/__init__.py — Scanner registry
• docs/agents/architecture.md — Existing architecture docs

Acceptance criteria

• Mermaid diagram renders correctly on GitHub
• Diagram covers the main scan flow from CLI to output
• Existing documentation is not duplicated (diagram complements text)