fix(tools): preserve nil-env semantics in script_shell so child inherits parent env

When `t.env` is nil (no toolset env wired), the prior fix changed `cmd.Env=nil` (Go inherits parent env) to a non-nil empty slice, stripping PATH/HOME/everything from the subprocess. Expand nil to `os.Environ()` before the per-call clone so the inherit-parent semantics are preserved.

Addresses Copilot review feedback on the prior fix in this branch.

Author: tdabasinskas

pkg/tools/builtin/script_shell.go

@@ -142,14 +142,16 @@ func (t *ScriptShellTool) execute(ctx context.Context, toolConfig *latest.Script
 
 	cmd := exec.CommandContext(ctx, shell, append(argsPrefix, toolConfig.Cmd)...)
 	cmd.Dir = toolConfig.WorkingDir
-	// `t.env` is shared across every concurrent invocation of execute().
-	// Aliasing it into cmd.Env and then appending the per-call params
-	// would mutate the shared backing array whenever cap > len, racing
-	// with sibling tool calls and silently leaking one call's params
-	// into another's environment. Materialise a per-call slice with
-	// exact capacity so each subprocess sees only its own params.
-	envCopy := make([]string, len(t.env), len(t.env)+len(params))
-	copy(envCopy, t.env)
+	// Per-call clone: appending onto t.env would mutate the shared
+	// backing array under concurrent calls. Expand nil to os.Environ()
+	// so a nil t.env still inherits the parent env (a non-nil empty
+	// slice would strip it).
+	base := t.env
+	if base == nil {
+		base = os.Environ()
+	}
+	envCopy := make([]string, len(base), len(base)+len(params))
+	copy(envCopy, base)
 	for key, value := range params {
 		if value != nil {
 			envCopy = append(envCopy, fmt.Sprintf("%s=%v", key, value))