feat(#1944): pi extension — installable ExtensionAPI host-plugin with reachability proof (#1965)

* 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)

tests/pi-extension-reachability.test.cjs

@@ -0,0 +1,59 @@
+'use strict';
+
+/**
+ * pi extension reachability test — ADR-1239 Phase D / #1944.
+ *
+ * Proves the pi extension is keystone-WIRED: the registered /gsd command
+ * handler dispatches through the GSD command-routing hub and returns a result
+ * (not just a registration on a mock). This is the "user can invoke X" proof.
+ */
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+
+const gsdPiExtension = require('../pi/gsd.cjs');
+
+function mockPi() {
+  const recorded = { commands: {}, tools: {}, events: [] };
+  return {
+    registerCommand(name, def) { recorded.commands[name] = def; },
+    registerTool(def) { if (def && def.name) recorded.tools[def.name] = def; },
+    on(event) { recorded.events.push(event); },
+    _recorded: recorded,
+  };
+}
+
+test('the pi extension registers /gsd + gsd_invoke + tool_call via ExtensionAPI', () => {
+  const pi = mockPi();
+  gsdPiExtension(pi);
+  assert.ok(pi._recorded.commands['gsd'], 'registers /gsd command');
+  assert.ok(pi._recorded.tools['gsd_invoke'], 'registers gsd_invoke tool');
+  assert.ok(pi._recorded.events.includes('tool_call'), 'subscribes to tool_call');
+});
+
+test('REACHABILITY: the /gsd handler dispatches through the engine hub (keystone wired)', async () => {
+  const pi = mockPi();
+  gsdPiExtension(pi);
+  // Invoke the registered /gsd handler — it must dispatch through createHub
+  // and return a JSON result (not throw). This is the keystone-wired proof.
+  const result = await pi._recorded.commands['gsd'].execute({
+    family: 'query',
+    subcommand: 'help',
+  });
+  assert.equal(typeof result, 'string', '/gsd handler returns a string result');
+  const parsed = JSON.parse(result);
+  assert.ok(parsed !== null && typeof parsed === 'object',
+    '/gsd dispatch produced a result object (the engine was reached)');
+});
+
+test('REACHABILITY: the gsd_invoke tool dispatches through the engine hub', async () => {
+  const pi = mockPi();
+  gsdPiExtension(pi);
+  const result = await pi._recorded.tools['gsd_invoke'].execute();
+  assert.equal(typeof result, 'string');
+  JSON.parse(result); // must be valid JSON (engine was reached)
+});
+
+test('gsdPiExtension throws without pi ExtensionAPI (fail-closed)', () => {
+  assert.throws(() => gsdPiExtension(null), /ExtensionAPI is required/);
+});