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/configRoutes.ts

@@ -58,6 +58,7 @@ function parseConfigRequest(value: unknown): PiWebConfig {
   const allowedHosts = value["allowedHosts"];
   const shortcuts = value["shortcuts"];
   const plugins = value["plugins"];
+  const spawnSessions = value["spawnSessions"];
   if (host !== undefined) {
     if (typeof host !== "string") throw new Error("PI WEB config host must be a string");
     config.host = host;
@@ -69,6 +70,10 @@ function parseConfigRequest(value: unknown): PiWebConfig {
   if (allowedHosts !== undefined) config.allowedHosts = parseAllowedHostsRequest(allowedHosts);
   if (shortcuts !== undefined) config.shortcuts = parseShortcutsRequest(shortcuts);
   if (plugins !== undefined) config.plugins = parsePluginsRequest(plugins);
+  if (spawnSessions !== undefined) {
+    if (typeof spawnSessions !== "boolean") throw new Error("PI WEB config spawnSessions must be a boolean");
+    config.spawnSessions = spawnSessions;
+  }
   return config;
 }
 
@@ -106,6 +111,7 @@ function piWebConfigEnvOverrides(env: NodeJS.ProcessEnv): PiWebConfigEnvOverride
     host: isEnvSet(env["PI_WEB_HOST"]),
     port: isEnvSet(env["PI_WEB_PORT"]) || isEnvSet(env["PORT"]),
     allowedHosts: isEnvSet(env["PI_WEB_ALLOWED_HOSTS"]),
+    spawnSessions: isEnvSet(env["PI_WEB_SPAWN_SESSIONS"]),
   };
 }