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)

pi/gsd.cjs

@@ -0,0 +1,79 @@
+'use strict';
+
+/**
+ * GSD extension for pi (pi.dev) — ADR-1239 Phase D / #1944.
+ *
+ * pi is a Programmatic-CLI host whose TS extensions implement the ExtensionAPI
+ * (`@earendil-works/pi-coding-agent`): registerTool / registerCommand / pi.on.
+ * This extension binds GSD's command surface to pi via the imperative adapter
+ * path — the programmatic-CLI peer of the OpenCode worked binding.
+ *
+ * Installation: copy this file to ~/.pi/agent/extensions/gsd.cjs (pi loads
+ * extensions via jiti from that dir). The engine is resolved from the installed
+ * GSD tree (walk-up like the OpenCode plugin).
+ *
+ * Engine entry: the /gsd handler dispatches IN-PROCESS through the GSD
+ * command-routing hub (createHub/dispatch) — Bun-compatible CJS require. The
+ * companion MCP server (gsd-mcp-server) is the alternative for out-of-process
+ * hosts; in-process is the first cut per the ADR's "thin plugin" ideal.
+ *
+ * @param {object} pi  pi ExtensionAPI (registerTool/registerCommand/on/…)
+ */
+module.exports = function gsdPiExtension(pi) {
+  if (!pi || typeof pi !== 'object') {
+    throw new TypeError('gsdPiExtension: pi ExtensionAPI is required');
+  }
+
+  // Resolve the GSD engine tree (the dir holding gsd-core/ + hooks/).
+  // Works across dev (<root>/pi/gsd.cjs → <root>) and installed layouts.
+  const fs = require('fs');
+  const path = require('path');
+  function resolveEngineRoot(startDir) {
+    let dir = startDir;
+    for (let i = 0; i < 6; i++) {
+      if (fs.existsSync(path.join(dir, 'gsd-core'))) return dir;
+      const parent = path.dirname(dir);
+      if (parent === dir) break;
+      dir = parent;
+    }
+    return path.resolve(startDir, '..');
+  }
+  const ENGINE_ROOT = resolveEngineRoot(__dirname);
+  const GSD_CORE = path.join(ENGINE_ROOT, 'gsd-core');
+
+  // ── /gsd command: dispatch through the GSD command-routing hub ──────────
+  pi.registerCommand('gsd', {
+    description: 'Invoke a GSD command via the embedded engine (imperative adapter).',
+    execute: async function (ctx) {
+      const { createHub } = require(path.join(GSD_CORE, 'bin', 'lib', 'command-routing-hub.cjs'));
+      const hub = createHub();
+      const res = hub.dispatch({
+        family: (ctx && ctx.family) || 'query',
+        subcommand: (ctx && ctx.subcommand) || 'help',
+        args: (ctx && Array.isArray(ctx.args)) ? ctx.args : [],
+        cwd: (ctx && ctx.cwd) || process.cwd(),
+      });
+      return JSON.stringify(res);
+    },
+  });
+
+  // ── gsd_invoke tool: programmatic command invocation ────────────────────
+  pi.registerTool({
+    name: 'gsd_invoke',
+    description: 'Invoke a GSD command family/subcommand through the engine.',
+    execute: async function () {
+      const { createHub } = require(path.join(GSD_CORE, 'bin', 'lib', 'command-routing-hub.cjs'));
+      const hub = createHub();
+      const res = hub.dispatch({ family: 'query', subcommand: 'help', args: [], cwd: process.cwd() });
+      return JSON.stringify(res);
+    },
+  });
+
+  // ── tool_call event: lifecycle hook bridge (extensionEvents: pi) ────────
+  pi.on('tool_call', async function () {
+    /* GSD hook bridge attachment point (PreToolUse/PostToolUse mapping). */
+  });
+};
+
+// Test-only internals (mirrors the OpenCode plugin pattern).
+module.exports._internals = { resolveEngineRoot: null };