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/TODO.md

@@ -57,7 +57,7 @@ This canonical checklist is the single source of truth for ongoing work. It mirr
   - Create: [bash.initramfs_create_cpio()](../scripts/lib/initramfs.sh:691)
   - Validate: [bash.initramfs_validate()](../scripts/lib/initramfs.sh:820)
 - [ ] QEMU / cloud-hypervisor smoke tests
-  - Test suite: [bash.testing_run_all()](../scripts/lib/testing.sh:299)
+  - Test runner: [runit.sh](../runit.sh)
 - [ ] Kernel embed path and versioning sanity
   - Embed config: [bash.kernel_modify_config_for_initramfs()](../scripts/lib/kernel.sh:130)
   - Full version logic: [bash.kernel_get_full_version()](../scripts/lib/kernel.sh:14)