[codex] JVM GC interview notes

[loulanyue]

Summary

• Add JVM GC guidance covering G1, ZGC, Full GC triggers, GC log reading, and tuning order.
• Keep the content aligned with agent_java_offer's backend interview focus while preserving interview-notes structure.

Target file

• 02-java/jvm/Java垃圾回收.md

Validation

• node scripts/run-maintenance-checks.mjs

Closes #96