docs: Add a concurrency section to AGENTS.md

[runningcode]

📜 Description

Adds a Concurrency section to AGENTS.md, next to the existing exception-handling guidance. The core rule: never hold a lock across a call that can re-enter the SDK — read what you need under the lock, release it, then make the call.

Also covers the second-lock escape hatch for compound operations that were only atomic because the lock spanned them, plus two supporting rules (volatile for cross-thread fields, and reading mutable shared state once per operation).

💡 Motivation and Context

Requested by @buenaflor while reviewing #6007.

Two production ANRs came from the same mistake. In #6007, AppStartExtension held its lock while calling finish() on the app start transaction; finishing captures synchronously and runs PerformanceAndroidEventProcessor, which takes its own lock and then calls back into AppStartExtension — opposite lock order, main thread blocked, ANR. JAVA-553 was the same shape in ReplayIntegration. The volatile and single-read rules come from #6006, where AppStartMetrics.appStartType was read three times in one pass and could disagree with itself.

The exception-handling section already establishes the format for this kind of repository-wide rule, so this follows it: the rule, why it fails, a wrong/right code pair, and the scope.

💚 How did you test it?

Documentation only. ./gradlew spotlessApply is clean.

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.
• Public API changes reviewed by another Mobile SDK team member or implemented according to the develop docs spec.

🔮 Next steps

Independent of #6007 — this documents the rule, that PR fixes the instance.

#skip-changelog