Add AGENTS.md

[dingsdax]

The repository-docs standard requires every SDK repo to have an AGENTS.md at the repository root. sentry-php currently has no AGENTS.md, meaning AI agents working in this repo have no architecture context, no testing conventions, no build instructions, and no SDK-specific standard overrides. The file that makes agentic contributions accurate and consistent.

What needs to be done

1. Create AGENTS.md at the repo root. Use agents-md skill.
2. Create CLAUDE.md as a symlink: ln -s AGENTS.md CLAUDE.md
3. Fill in all SDK-specific sections:
   • Package manager and key commands (Composer, PHPUnit, PHP CS Fixer, PHPStan/Psalm, phpbench for benchmarks)
   • Architecture overview (core SDK structure, Hub/Client/Transport layer, integrations in src/, stubs in stubs/)
   • Development workflow (composer install, phpunit, static analysis, phpbench)
   • Testing conventions (PHPUnit, test file naming, psalm type checking)
   • Standards overrides (any known deviations from the SDK standards)
4. Verify accuracy: have an AI tool attempt a small contribution using the file as context before merging

[linear]

PHP-68