Update AGENTS.md with pre-upload quality gates and decision rubrics

- Add Pre-Upload Quality Gate checklist covering ktfmt, Checkstyle import
  ordering, and local ErrorProne release variant checks (runErrorProneRelease)
  to prevent presubmit CI failures.
- Add Multi-Commit Stack Hygiene guidelines to eliminate intermediate test
  churn and ensure self-contained, atomic CLs.
- Add Decision Framework distinguishing Core Architecture Fixes vs. Device
  Quirks vs. Test Assumption Cleanup.
- Add Safety & Code Path Auditing Protocol for shared UseCase infrastructure.
- Add Skill: CameraX Agent Guidelines & Experience Capture to continuously
  distill session experience into updated documentation.

Bug: 550078731
Test: None (Documentation update)
Change-Id: Iba9f306632eb9638402d650171f1dab661b52df1