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/NOTES.md

@@ -176,9 +176,12 @@ Stage System and Incremental Rebuilds
   - Shows stage status before/after marker removal; no --rebuild-from is passed by default (relies on markers only).
 - Manual minimal rebuild:
   - Remove relevant .done files, e.g.: initramfs_create.done initramfs_test.done validation.done
-  - Rerun: DEBUG=1 ./scripts/build.sh --skip-tests
+  - Rerun: DEBUG=1 ./scripts/build.sh
 - Show status:
   - ./scripts/build.sh --show-stages
+- Test built kernel:
+  - ./runit.sh --hypervisor qemu
+  - ./runit.sh --hypervisor ch --disks 5 --reset
 
 Key Decisions (current)
 - Firmware selection for initramfs comes exclusively from [config/firmware.conf](config/firmware.conf); firmware hints in modules.conf are ignored to avoid duplication/mismatch.