docs: update README Quickstart — .NET 9.0 + modern commands (#278 Task 7)

[jsboige]

Summary

• Replace obsolete .NET 7.0 prerelease instructions with clone-and-run quickstart
• Add separate sections for developers (full build) and end users (runtime only)
• Update CardPen URLs to current values (post-fix(config): ReleaseCardpenUrl points to 404 — Pages workflow URL mismatch #279 fix: GitHub Pages root, myia.io)
• Condense macOS/Linux troubleshooting
• Add note about auto-generated config on first run

Test plan

• Verify rendered markdown looks correct
• Confirm commands are accurate for .NET 9.0

Part of Epic #278 (Task 7/7).

🤖 Generated with Claude Code

[clusterManager-Myia]

Automated review.

Verdict: LGTM ✅

• README modernized from .NET 7.0 to .NET 9.0
• Clear Quick Start sections split for developers vs end users
• Playwright installation instructions included
• Updated Cardpen URLs (release + debug)
• macOS/Linux section simplified and cleaned up

No issues found.

— Hermes (myia-po-2026) [CRON:review-pr 18:36Z]

[jsboige]

LGTM ✅ — README modernisé proprement.

Améliorations notables :

• .NET 7.0 prerelease → .NET 9.0 SDK quickstart cohérent avec le code actuel
• Deux quickstarts séparés (devs vs end users) — bonne pédagogie
• URL CardPen Release /Argumentum/index.html cohérente avec fix fix(config): ReleaseCardpenUrl points to 404 — Pages workflow URL mismatch #279 (PR fix(config): correct ReleaseCardpenUrl — GitHub Pages root #281)
• Note auto-génération AssetConverterConfig.json au premier run — important pour onboard
• macOS/Linux condensé sans perte d'info utile

Note mineure non bloquante : le diff JSON exemple ligne ~104 montre https://argumentum.myia.io/index.html mais le LocalCardpenUrl dans WebBasedGeneratorConfig.cs est en http:// (cf. CLAUDE.md). Cohérence à vérifier en follow-up éventuel, mais l'exemple JSON est illustratif, donc OK.

Hermes ✅, GitGuardian ✅. Merge.