Fix non-hermetic Pi config test that passed locally but failed CI (XDG_CONFIG_HOME)

The Pi config loader resolves the user config base as
(XDG_CONFIG_HOME ?? <HOME>/.config)/cortexkit/..., but loadPiConfig's tests wrote
the user fixture under <HOME>/.config and set only HOME — never pinning
XDG_CONFIG_HOME. On a dev machine where XDG_CONFIG_HOME is unset the fallback hits
the temp HOME and the tests pass; on CI (which exports its own XDG_CONFIG_HOME)
the loader looks there instead, finds no fixture, and reads schema defaults, so
ctx_reduce_enabled/memory.enabled/env-substitution assertions failed. This was
introduced with the CortexKit config-location cutover and only surfaced in CI
(the local full gate was a false green because this machine has XDG unset).

withHome() now also pins XDG_CONFIG_HOME to <HOME>/.config and afterEach restores
it. The OpenCode config test already pinned XDG; this brings Pi to parity.
Verified the suite passes with XDG_CONFIG_HOME both set (CI shape) and unset.

Co-authored-by: Alfonso [Magic Context] <288211368+alfonso-magic-context@users.noreply.github.com>

Author: Ufuk Altinok

packages/pi-plugin/src/config/index.test.ts

@@ -8,6 +8,7 @@ import { loadPiConfig, loadPiConfigDetailed } from "./index";
 
 const tempRoots: string[] = [];
 const originalHome = process.env.HOME;
+const originalXdgConfigHome = process.env.XDG_CONFIG_HOME;
 
 function makeTempRoot(prefix: string): string {
 	const path = mkdtempSync(join(tmpdir(), prefix));
@@ -17,6 +18,12 @@ function makeTempRoot(prefix: string): string {
 
 function withHome(home: string): void {
 	process.env.HOME = home;
+	// The user config base is `(XDG_CONFIG_HOME ?? <HOME>/.config)/cortexkit/...`.
+	// writeUserConfig() writes under `<HOME>/.config`, so pin XDG_CONFIG_HOME to
+	// match — otherwise a CI runner that exports its own XDG_CONFIG_HOME makes the
+	// loader look elsewhere and these tests read schema defaults (the green-on-my-
+	// machine / red-in-CI hermeticity gap this guards against).
+	process.env.XDG_CONFIG_HOME = join(home, ".config");
 }
 
 function writeConfig(path: string, text: string): void {
@@ -55,6 +62,11 @@ afterEach(() => {
 	} else {
 		process.env.HOME = originalHome;
 	}
+	if (originalXdgConfigHome === undefined) {
+		delete process.env.XDG_CONFIG_HOME;
+	} else {
+		process.env.XDG_CONFIG_HOME = originalXdgConfigHome;
+	}
 
 	for (const path of tempRoots.splice(0)) {
 		rmSync(path, { recursive: true, force: true });