gemini extensions install <url or path> copies the extension directory alone. plugin/gemini/ runs ../bin/hook.py, so today it works only through gemini extensions link plugin/gemini from a checkout. That matches how the CLI is installed today (cargo install --path), but not a released CLI (#254).
Two ways out: ship hook.py inside the extension when a release is cut (a copy, with a test that it is identical), or a gemini-extension.json at the root of a release archive that holds both. Either way, add an install-from-path run to plugin/tests/e2e_harnesses.py.