docs: add cascade-themed mermaid diagrams to readme and docs

[joshua-temple]

Problem: the root README and docs explained the pipeline with ASCII box diagrams, and the docs mermaid theming did not reflect the cascade teal brand.

Fix:

• README "How it works" ASCII flow replaced with a Mermaid flowchart (manifest to generate-workflow to generated workflows to Orchestrate to Promote cascade to Release lifecycle). Removed the stale TODO image placeholder. Themed with a per-diagram init directive (theme base + teal/copper themeVariables) tuned to read on both GitHub light and dark backgrounds.
• docs astro-mermaid given cascade-teal mermaidConfig themeVariables (teal edges, borders, cluster outlines; copper secondary accent), kept autoTheme so node fills/text stay legible in both Starlight light and dark modes.
• Converted ASCII diagrams to Mermaid in architecture.md (System Overview, Primary/Satellite multi-repo) and workflows.md (Orchestrate flow, Promote flow). Existing state-machine diagram preserved.

Verification:

• All Mermaid blocks (README + 4 docs) parse via mermaid-cli.
• cd docs && npm run build succeeds (9 pages); astro-mermaid transforms the blocks and the teal themeVariables are baked into the page output.