Proves the Programmatic-CLI reference binding for pi via the ExtensionAPI
imperative adapter (#1682 AC): createImperativeAdapter({runtime:'pi'}) classifies
as imperative + composes the registry; pi axes (imperative + bun) classify as
'programmatic-cli'; the reference pi host-plugin binds GSD via ExtensionAPI
(registerCommand /gsd + registerTool gsd_invoke + on tool_call).
Reference plugin (tests/fixtures/pi-host-plugin.cjs) is CJS + mock-friendly so
it is behaviorally testable without a live pi runtime. Full --pi installable-
runtime integration (descriptor + installer + golden parity 16→17) is a larger
follow-up — intentionally NOT added to the runtime registry here.
