Update README: replace ASCII diagram with Mermaid flowchart

[ProductOfAmerica]

Description

Replaced the legacy ASCII architecture diagram with a modern, responsive Mermaid.js flowchart.

Why?

• Scalability: The previous ASCII diagram was visually ambiguous regarding whether tasks branched from "Kits" or "Build Site."
• Readability: Mermaid renders natively in GitHub with better contrast and support for light/dark modes.
• Maintainability: Easier for future contributors to update the workflow by editing text rather than manual ASCII drawing.

Changes

• Visual clarification of the /ck:make parallel execution phase.
• Updated node shapes to better represent the "Build Site" (state/database) vs. "Kits" (documents).
• Applied a dark-themed styling configuration to match the project's aesthetic.

Rendered Example: