Fix fork-mode lifecycle test and default test model to gpt-5.6-luna

The fork test asked the model to call subagent with fork: true and no
agent, which the tool schema has never supported - it only passed when a
model improvised. It now spawns a real fork-mode agent (test-fork with
session-mode: fork).

The harness default model is now openai-codex/gpt-5.6-luna. The previous
active-session-model fallback broke under pi -ne, where extension-registered
providers do not resolve.

test/integration/harness.ts

@@ -73,13 +73,14 @@ const EXTENSION_SOURCE = join(PROJECT_ROOT, "pi-extension", "subagents", "index.
 
 // ── Configuration ──
 
-/** Model used for integration tests. Override with PI_TEST_MODEL env var. */
-const activePiModel =
-  process.env.PI_PROVIDER && process.env.PI_MODEL
-    ? `${process.env.PI_PROVIDER}/${process.env.PI_MODEL}`
-    : undefined;
-export const TEST_MODEL =
-  process.env.PI_TEST_MODEL ?? activePiModel ?? "anthropic/claude-haiku-4-5";
+/**
+ * Model used for integration tests. Override with PI_TEST_MODEL env var.
+ *
+ * The default must be a provider that exists under `pi -ne`: startPi() launches
+ * with extensions disabled, so extension-registered providers (and therefore
+ * the parent session's active model) may not resolve in the child.
+ */
+export const TEST_MODEL = process.env.PI_TEST_MODEL ?? "openai-codex/gpt-5.6-luna";
 
 /** Per-test timeout in ms. Override with PI_TEST_TIMEOUT env var. */
 export const PI_TIMEOUT = Number(process.env.PI_TEST_TIMEOUT ?? "120000");