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/PROMPT.md

@@ -107,7 +107,7 @@ Diagnostics-first workflow (strict)
 - For any failure, first collect specific logs:
   - Enable DEBUG=1 for verbose logs.
   - Re-run only the impacted stage if possible:
-    - Example: rm -f .build-stages/validation.done && DEBUG=1 ./scripts/build.sh --skip-tests
+    - Example: rm -f .build-stages/validation.done && DEBUG=1 ./scripts/build.sh
 - Use existing diagnostics:
   - Branding debug lines: [bash.initramfs_finalize_customization()](scripts/lib/initramfs.sh:575)
   - Validation debug lines (input, PWD, PROJECT_ROOT, INSTALL_DIR, resolved): [bash.initramfs_validate()](scripts/lib/initramfs.sh:799)
@@ -119,12 +119,15 @@ Common tasks and commands
   - DEBUG=1 ./scripts/build.sh
 - Minimal rebuild of last steps:
   - rm -f .build-stages/initramfs_create.done .build-stages/initramfs_test.done .build-stages/validation.done
-  - DEBUG=1 ./scripts/build.sh --skip-tests
+  - DEBUG=1 ./scripts/build.sh
 - Validation only:
   - rm -f .build-stages/validation.done
-  - DEBUG=1 ./scripts/build.sh --skip-tests
+  - DEBUG=1 ./scripts/build.sh
 - Show stage status:
   - ./scripts/build.sh --show-stages
+- Test built kernel:
+  - ./runit.sh --hypervisor qemu
+  - ./runit.sh --hypervisor ch --disks 5
 
 Checklists and helpers
 
@@ -154,7 +157,7 @@ C) Minimal rebuild after zinit/init/modules.conf changes
   - Stage status is printed before/after marker removal; the helper avoids --rebuild-from by default to prevent running early stages.
 - Manual fallback:
   - rm -f .build-stages/initramfs_create.done .build-stages/initramfs_test.done .build-stages/validation.done
-  - DEBUG=1 ./scripts/build.sh --skip-tests
+  - DEBUG=1 ./scripts/build.sh
 
 D) INITRAMFS_ARCHIVE unbound during kernel build stage
 - stage_kernel_build now defaults INITRAMFS_ARCHIVE if unset: