Add COLIBRI_STAGE_TEST_AGENT with dev/release defaults so validation builds can include colibri-test-agent while production/release operator USB images omit it by default. Keep poudriere guidance test-friendly and document binary roles in BUILD.md.\n\nValidation: sh -n build.sh scripts/stage-colibri-iso.sh live/operator-session/colibri-live-rebuild; ./scripts/check-format.sh; ./scripts/test-release-gate.sh; build.cfg default/override checks.
