Remove stale showcase code and align repository docs

[manynames3]

Summary

• removes the unreferenced HomeCockpit component and the exports used only by that dead path
• corrects README claims about the active runbook/compliance surfaces and dashboard loading behavior
• documents the frontend regression test in the README, reviewer guide, testing guide, and architecture evidence

Validation

• 34 passed backend tests
• npm test
• npm run typecheck
• npm run lint
• npm run build
• terraform fmt -check -recursive infra/terraform
• terraform validate
• local Markdown link validation
• git diff --check

No runtime behavior or deployment configuration is changed.

[gemini-code-assist]

Code Review

This pull request introduces a new frontend regression test (npm test) to verify showcase risk-score regressions, updating relevant documentation across the repository (including README.md, docs/architecture-notes.md, docs/reviewer-guide.md, and docs/testing.md) to reflect this addition. Additionally, unused showcase-related constants and helper functions have been cleaned up from apps/web/lib/showcase.ts. There are no review comments, so I have no feedback to provide.

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