Streamline PHP SDK first-visit experience

[rmcdaniel]

Summary

• replace the long root README with a concise PHP, Laravel, and Symfony entry point while preserving the full reference under docs/
• remove retired alternate-CI routing, scheduled link diagnostics, deployment requalification, and the duplicate published framework workflow
• keep the PHP and framework compatibility matrices, Laravel transition checks, replay/codec corpus, analysis, package smoke, public-boundary scan, and pre-deploy browser layout check
• refresh package metadata for the stable Cloud and self-hosted Server product surface

Part of durable-workflow/.github#117.

Validation

• 434 PHPUnit tests / 2,128 assertions
• PHPStan
• 153-file PHP syntax pass
• Avro Value regression budget
• 38 codec fixtures, 21 replay fixtures, and 83 corpus-policy tests
• docs example, portal, API, analytics, link, and browser-failure checks
• desktop and mobile portal render review
• central GitHub action policy
• Composer package and public-boundary validation