Add architecture visualizer canvas

[dbroeglin]

Adds a project-scoped Copilot canvas so users can inspect the copilot-experiments architecture directly in the app instead of relying only on static docs. The canvas renders an interactive layered diagram with focusable components, design invariants, and details for key files and flows.

Approach

• Registers the architecture-visualizer project extension with canvas open input validation and get_architecture / focus_component actions.
• Serves a self-contained loopback HTML renderer that uses the app theme variables and does not require external runtime dependencies.
• Adds a local layered graph layout helper that computes lane, node, label, and rounded orthogonal edge geometry for a cleaner side-panel rendering.

Validation

• node --check .github\extensions\architecture-visualizer\graph-layout.mjs
• node --check .github\extensions\architecture-visualizer\extension.mjs