Fix opencode not found: symlink installer binary to /usr/local/bin

The opencode curl installer puts the binary in /root/.local/bin which
isn't on PATH for the nextjs user. Add a symlink to /usr/local/bin
after install. Also ensure /usr/local/bin is always in the PATH
passed to spawned agent processes.

apps/harness/src/lib/agent-env.ts

@@ -14,6 +14,12 @@ export async function buildAgentEnv(
 ): Promise<NodeJS.ProcessEnv> {
   const env: NodeJS.ProcessEnv = { ...process.env, TERM: "xterm-256color" };
 
+  // Ensure global bin paths are available (npm -g, opencode installer)
+  const path = env.PATH || "/usr/local/bin:/usr/bin:/bin";
+  if (!path.includes("/usr/local/bin")) {
+    env.PATH = `/usr/local/bin:${path}`;
+  }
+
   const providersToInject =
     config.runtime === "opencode"
       ? Object.keys(PROVIDER_ENV_VARS)