docs: update CONTRIBUTING.md and QUICKSTART.md to match Astro stack

[jongio]

Summary

CONTRIBUTING.md and QUICKSTART.md still reference the pre-migration React/Vite/Vitest stack that was replaced with Astro in PR #9. This actively misleads contributors who follow the documented setup steps.

Problems Fixed

File	Issue
CONTRIBUTING.md	Listed 8 non-existent npm scripts (pnpm test, pnpm lint, pnpm type-check, etc.)
CONTRIBUTING.md	Referenced non-existent files (ExtensionCard.tsx, App.tsx, main.tsx, index.css)
CONTRIBUTING.md	Described React coding standards (hooks, functional components) instead of Astro
CONTRIBUTING.md	Project structure showed shadcn/ui, icons/, test/, types/ directories that don't exist
CONTRIBUTING.md	CI workflow description said "Lint, type-check, test, build" instead of actual pipeline
QUICKSTART.md	Said dependencies include "React 19, Vite, and testing tools"
QUICKSTART.md	Dev server URL was localhost:5173 (Vite default) instead of localhost:4321 (Astro default)
QUICKSTART.md	Listed non-existent commands (pnpm test, pnpm lint, pnpm format, etc.)
QUICKSTART.md	Troubleshooting referenced "Vite" behavior instead of Astro

Changes

• Updated development commands table to match actual package.json scripts
• Replaced React coding standards with Astro component guidance
• Updated "Adding a New Extension" step 6 to reference ExtensionShowcase.astro instead of ExtensionCard.tsx
• Corrected project structure to reflect actual src/ layout
• Fixed dev server port and dependency descriptions
• Updated quality standards checklist to match actual CI checks

Validation

• pnpm build passes before and after changes

[jongio]

Superseded by swarm PR #88. Will be re-addressed in a follow-up swarm session.