Summary

• make the backend test gate explicit by switching documented and CI test commands to zig build test -Dembed-ui=false -Dbuild-ui=false --summary all
• fix the clean-worktree failure mode where zig build test -Dbuild-ui=false still expected prebuilt embedded UI assets because UI embedding remained enabled
• update nullclaw/nullbuilder main and v1 with reusable workflow hooks for Node setup, target matrices, generated asset prebuilds, E2E commands, source archives, target-specific check names, and the shared Zig setup action
• centralize NullHub CI and release workflows on nullbuilder@v1, including UI asset preparation, E2E, ReleaseSmall artifacts, source archive, and Docker publishing
• update the build panic text and docs so contributors are told about the backend-only -Dembed-ui=false path

Validation

• rm -rf ui/node_modules ui/build .generated_ui_assets.zig && zig build test -Dembed-ui=false -Dbuild-ui=false --summary all
• npm --prefix ui ci --no-audit --no-fund
• npm --prefix ui run build
• bash tests/test_e2e.sh
• zig fmt --check src/
• ruby -e 'require "yaml"; Dir[".github/workflows/*.yml"].each { |f| YAML.load_file(f); puts "ok #{f}" }'
• ruby -e 'require "yaml"; require "json"; [".github/workflows/ci.yml", ".github/workflows/release.yml"].each { |f| y=YAML.load_file(f); jobs=y.fetch("jobs"); jobs.each_value { |j| next unless j["with"] && j["with"]["targets_json"]; JSON.parse(j["with"]["targets_json"]) }; puts "json ok #{f}" }'
• git diff --check

Notes

• nullbuilder@v1 now points at d188f08, which includes the hooks used by this PR
• NullHub keeps its release target set explicit while using the shared release workflow