feat: let agents start new sessions via spawn_session tool

Add a project-scoped spawn_session tool so agents can dispatch new,
independent sessions (ralph loops, long-plan chaining). Spawned sessions
are constrained to a workspace/worktree of the same registered project,
appear in the session list immediately via a new session.created event,
and the capability is on by default with a Settings -> Session daemon
toggle (spawnSessions / PI_WEB_SPAWN_SESSIONS).

Note: adds a session daemon code path, so pi-web-sessiond.service must be
restarted manually for the server side to take effect.

src/server/sessions/spawnSessionTool.test.ts

@@ -0,0 +1,37 @@
+import type { ExtensionContext } from "@earendil-works/pi-coding-agent";
+import { describe, expect, it, vi } from "vitest";
+import { createSpawnSessionToolDefinition } from "./spawnSessionTool.js";
+
+// The spawn tool's execute() never reads ctx, so an empty stub is sufficient.
+// eslint-disable-next-line @typescript-eslint/consistent-type-assertions -- test stub; execute() does not use ctx.
+const ctx = {} as ExtensionContext;
+
+describe("createSpawnSessionToolDefinition", () => {
+  it("passes the spawning cwd and params to the spawn callback and reports success", async () => {
+    const spawn = vi.fn(() => Promise.resolve({ sessionId: "new-1", cwd: "/repos/a-feature" }));
+    const tool = createSpawnSessionToolDefinition("/repos/a", { spawn });
+
+    const result = await tool.execute("call-1", { prompt: "do the thing", cwd: "/repos/a-feature" }, undefined, undefined, ctx);
+
+    expect(spawn).toHaveBeenCalledWith({ spawningCwd: "/repos/a", prompt: "do the thing", cwd: "/repos/a-feature" });
+    expect(result.details).toEqual({ sessionId: "new-1", cwd: "/repos/a-feature" });
+    expect(result.content[0]).toMatchObject({ type: "text", text: "Started session new-1 in /repos/a-feature." });
+  });
+
+  it("defaults cwd to undefined so the service falls back to the spawning cwd", async () => {
+    const spawn = vi.fn(() => Promise.resolve({ sessionId: "new-2", cwd: "/repos/a" }));
+    const tool = createSpawnSessionToolDefinition("/repos/a", { spawn });
+
+    await tool.execute("call-2", { prompt: "continue" }, undefined, undefined, ctx);
+
+    expect(spawn).toHaveBeenCalledWith({ spawningCwd: "/repos/a", prompt: "continue", cwd: undefined });
+  });
+
+  it("propagates the spawn callback error so the agent loop reports it", async () => {
+    const spawn = vi.fn(() => Promise.reject(new Error("cwd must be a workspace of this project. Allowed: /repos/a")));
+    const tool = createSpawnSessionToolDefinition("/repos/a", { spawn });
+
+    await expect(tool.execute("call-3", { prompt: "x", cwd: "/elsewhere" }, undefined, undefined, ctx))
+      .rejects.toThrow("cwd must be a workspace of this project. Allowed: /repos/a");
+  });
+});