The default Shell is $SHELL, else /bin/sh

With no Shell set, pardes ran fish, which a box may not have and a user
may not use. The default is now the user's login shell ($SHELL), else
/bin/sh; an explicit Shell still overrides it. The tests pin their own
shell (snapshots bash, the python suites /bin/sh or bash) and pass with
SHELL unset; the Config report and docs say where the default comes from.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 3 insertions, 2 deletions

diff --git a/test/snapshot.zig b/test/snapshot.zig
index 2fa8e89c..c49e7645 100644
--- a/test/snapshot.zig
+++ b/test/snapshot.zig
@@ -562,8 +562,9 @@ fn runScript(arena: std.mem.Allocator, exe_z: [:0]const u8, script_path: []const
     // XDG directory that points outside this per-snapshot fake home.
     const config_home = try std.fmt.allocPrintSentinel(arena, "{s}/.config", .{home}, 0);
     _ = setenv("XDG_CONFIG_HOME", config_home, 1);
-    // ...and it is not empty: the DEFAULT shell is fish, whose prompt carries
-    // a hostname and whose greeting carries a version, so a golden taken
+    // ...and it is not empty: the DEFAULT shell is $SHELL, whatever this
+    // machine's user logs in with (fish's prompt carries a hostname and its
+    // greeting a version), so a golden taken
     // against it would be a golden for this machine. Every script here runs
     // bash — the .bashrc above pins its prompt to `$ ` — and the fish path is
     // covered by host_io.Shell's unit tests instead. This is an ordinary