feat(harness): gate uses externally-provided THT_SESSION id (BE-5)

When THT_SESSION env var is set, /nuova-domanda injects a kickoff that
tells the model to use the pre-created session id instead of running
`tht session new`. /riprendi-sessione and no-env-var paths unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

harness/.pi/extensions/gate/__tests__/gate_provided_session.test.js

@@ -0,0 +1,30 @@
+const test = require("node:test");
+const assert = require("node:assert");
+const { createFakePi } = require("./fake_pi_runtime.js");
+const installGate = require("../../tht-gate.js").default ?? require("../../tht-gate.js");
+
+test("con THT_SESSION il kickoff usa l'id fornito e NON crea la sessione", async () => {
+  process.env.THT_SESSION = "2026-06-27-100000-test";
+  try {
+    const { pi, ctx } = createFakePi();
+    installGate(pi);
+    await pi.emit("input", { source: "rpc", text: '/nuova-domanda "x"' });
+    const injected = await pi.emit("before_agent_start", { systemPrompt: "" });
+    // before_agent_start returns { systemPrompt: "<original>\n\n<kickoff>" }
+    const text = injected?.systemPrompt ?? "";
+    assert.match(text, /2026-06-27-100000-test/);
+    assert.doesNotMatch(text, /tht session new/);
+  } finally {
+    delete process.env.THT_SESSION;
+  }
+});
+
+test("senza THT_SESSION il kickoff contiene tht session new (comportamento attuale)", async () => {
+  delete process.env.THT_SESSION;
+  const { pi, ctx } = createFakePi();
+  installGate(pi);
+  await pi.emit("input", { source: "rpc", text: '/nuova-domanda "x"' });
+  const injected = await pi.emit("before_agent_start", { systemPrompt: "" });
+  const text = injected?.systemPrompt ?? "";
+  assert.match(text, /tht session new/);
+});