fix: make bash tool respect $SHELL (#3494)

Co-authored-by: opencode-agent[bot] <opencode-agent[bot]@users.noreply.github.com>
Co-authored-by: rekram1-node <rekram1-node@users.noreply.github.com>

packages/opencode/src/tool/bash.ts

@@ -183,7 +183,7 @@ export const BashTool = Tool.define("bash", {
     }
 
     const proc = spawn(params.command, {
-      shell: true,
+      shell: process.env.SHELL ?? true, // $SHELL if set, otherwise default shell
       cwd: Instance.directory,
       env: {
         ...process.env,