* feat(#1944): pi extension — installable ExtensionAPI host-plugin with reachability proof
The pi extension (pi/gsd.cjs) is a real, jiti-loadable ExtensionAPI module that
registers /gsd (dispatches through the GSD command-routing hub) + gsd_invoke tool
+ tool_call event. Engine entry: in-process CJS require (Bun-compatible). The
extension resolves the engine tree via walk-up (like the OpenCode plugin).
tests/pi-extension-reachability.test.cjs proves the /gsd handler DISPATCHES
through the hub (returns a JSON result) — the keystone-wired proof that the
command is user-reachable, not just registered on a mock (#1944 AC).
* docs(changeset): pi extension (#1965)
* fix(#1944): add docs-exempt marker to changeset (no standalone docs file)
