Harness-neutral, lighter wording for the optional local test-double agent
(colibri-test-agent), matching the colibri-side fake→sample rename. Only the
two references that named it 'fake-agent' (build.cfg comment, AGENTS.md
staging note); the unrelated /tmp/fake-usb example path in FIRSTBOOT.md is a
different context and left as-is.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
