version: show a spinner while it checks for updates

Checking for newer versions goes to the network and can take several
seconds with nothing on screen. Show "Checking for updates..." with a
spinner on the terminal until every check is done, then print the
results all at once. The spinner goes to stderr and only to a terminal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/version.bats

@@ -133,3 +133,18 @@ sibling_repo() {
   [[ "$output" == *"tools is at v1.1.0."* ]]
   [ "$(cat "$HOME/updated")" = "$HOME/.local/share/siblings/tools" ]
 }
+
+@test "version shows a spinner on a terminal while it checks, and clears it" {
+  run script -qec "bash '$CD/version'" /dev/null < /dev/null
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"Checking for updates..."* ]]
+  # Cleared, and the cursor shown again, before the results are printed
+  [[ "$output" == *$'\r\e[K\e[?25hCoreDesktop  2026.10.31'* ]]
+}
+
+@test "version shows no spinner when its output isn't a terminal" {
+  run bash "$CD/version"
+  [ "$status" -eq 0 ]
+  [[ "$output" != *"Checking"* ]]
+  [[ "$output" != *$'\e[?25l'* ]]
+}