build: remove testing.sh in favor of runit.sh; add claude.md reference

Replace inline boot testing with standalone runit.sh runner for clarity:
- Remove scripts/lib/testing.sh source and boot_tests stage from build.sh
- Remove --skip-tests option from build.sh and rebuild-after-zinit.sh
- Update all docs to reference runit.sh for QEMU/cloud-hypervisor testing
- Add comprehensive claude.md as AI assistant entry point with guidelines

Testing is now fully decoupled from build pipeline; use ./runit.sh for
QEMU/cloud-hypervisor validation after builds complete.

docs/review-rfs-integration.md

@@ -13,7 +13,6 @@ Key sourced libraries:
 - [initramfs.sh](scripts/lib/initramfs.sh)
 - [stages.sh](scripts/lib/stages.sh)
 - [docker.sh](scripts/lib/docker.sh)
-- [testing.sh](scripts/lib/testing.sh)
 
 Main stages executed (incremental via [stage_run()](scripts/lib/stages.sh:99)):
 1) alpine_extract, alpine_configure, alpine_packages