fix(embed): install @huggingface/transformers into codegraph's host node_modules

`embed`'s lazy install calls `execFileSync('npm', ['install', ...])` with
no `cwd`. Default cwd = `process.cwd()`, which is whatever directory the
user invoked codegraph from — typically NOT the directory where codegraph
itself is installed.

The resulting install lands in the user's project node_modules, but the
subsequent `await import('@huggingface/transformers')` resolves from
codegraph's own location, where the dep is missing. The user sees:

    [ENGINE_UNAVAILABLE]: @huggingface/transformers was installed
    but failed to load. Please check your environment.

…while their project's node_modules silently gains an unwanted dep.

Pin `cwd` to the host directory that contains @optave/codegraph (resolved
via `_require.resolve('@optave/codegraph/package.json')` then four
`path.dirname` hops). The install now lands in the same node_modules that
contains codegraph, so the dynamic import resolves correctly without
polluting the user's working directory.

Falls back to `cwd: undefined` (= process.cwd()) when @optave/codegraph
can't be resolved — typically a source-of-truth dev checkout — so the
existing test suite (which mocks the import to fail) continues to pass.

Closes #1175

Author: carlos-alm

src/domain/search/models.ts

@@ -1,8 +1,38 @@
 import { execFileSync } from 'node:child_process';
+import { createRequire } from 'node:module';
+import path from 'node:path';
 import { createInterface } from 'node:readline';
 import { info } from '../../infrastructure/logger.js';
 import { ConfigError, EngineError } from '../../shared/errors.js';
 
+const _require = createRequire(import.meta.url);
+
+/**
+ * Resolve the directory where `npm install` should run so the installed
+ * package ends up reachable by `await import(pkg)` from inside this module.
+ *
+ * Without a `cwd`, `execFileSync('npm', ['install', ...])` operates on
+ * `process.cwd()` — when the user runs codegraph against a repo that is *not*
+ * the directory where codegraph itself is installed, npm installs into the
+ * wrong `node_modules`, the dynamic import still fails, and the user gets
+ * `ENGINE_UNAVAILABLE: ... installed but failed to load`.
+ *
+ * Pin cwd to the directory that contains @optave/codegraph's `node_modules`
+ * so the install lands where Node's resolution algorithm will find it.
+ */
+function resolveNpmInstallCwd(): string | undefined {
+  try {
+    const pkgJsonPath = _require.resolve('@optave/codegraph/package.json');
+    // pkgJsonPath = <host>/node_modules/@optave/codegraph/package.json
+    // dirname x4: package.json → codegraph → @optave → node_modules → <host>
+    return path.dirname(path.dirname(path.dirname(path.dirname(pkgJsonPath))));
+  } catch {
+    // Source-of-truth checkout (no @optave/codegraph in node_modules) — fall back
+    // to process.cwd() so legacy behavior survives in tests.
+    return undefined;
+  }
+}
+
 export interface ModelConfig {
   name: string;
   dim: number;
@@ -104,12 +134,14 @@ export function getModelConfig(modelKey?: string): ModelConfig {
  * @internal Not part of the public barrel.
  */
 export function promptInstall(packageName: string): Promise<boolean> {
+  const installCwd = resolveNpmInstallCwd();
   if (!process.stdin.isTTY) {
     info(`Installing ${packageName} (optional dependency for semantic search)…`);
     try {
       execFileSync(NPM_BIN, ['install', '--no-save', packageName], {
         stdio: 'inherit',
         timeout: 300_000,
+        cwd: installCwd,
       });
       return Promise.resolve(true);
     } catch (err) {
@@ -131,6 +163,7 @@ export function promptInstall(packageName: string): Promise<boolean> {
           execFileSync(NPM_BIN, ['install', packageName], {
             stdio: 'inherit',
             timeout: 300_000,
+            cwd: installCwd,
           });
           resolve(true);
         } catch (err) {