Update AGENTS maintainer guide

[ilysenko]

Summary

• refresh AGENTS.md around the current Linux features architecture
• document local features, declarative resources/runtime hooks, package hooks, staged manifests, and disabled-by-default feature policy
• update source-of-truth sections for AppImage, read-aloud-linux, bootstrap/native targets, and patch descriptor layout

Validation

• git diff --check
• checked explicitly referenced paths exist
• documentation-only change; no runtime tests run locally