fix(#1356): distinguish 'tea not installed' from 'login missing'

Self-review catch. The ladder reported "no tea login named X exists" when tea
was simply absent -- a cause it never checked, sending the reader off to create a
login they have no tool to create. Same defect class as the one just filed as
#1357, so shipping a fresh instance of it in this PR would be poor form.

packages/mosaic/framework/tools/git/detect-platform.sh

@@ -353,6 +353,14 @@ get_gitea_login_for_host() {
                 echo "$canon"
                 return 0
             fi
+            # Say which of the two it is. "No such login" when tea is simply not
+            # installed is a diagnosis of a cause that was never checked, and it
+            # sends the reader off to create a login they cannot create.
+            if ! command -v tea >/dev/null 2>&1; then
+                echo "Error: git identity '$ident' requested (via $ident_src) for host '$host', but tea is not installed," >&2
+                echo "       so no login can be resolved. Refusing to guess an identity." >&2
+                return 1
+            fi
             echo "Error: git identity '$ident' requested (via $ident_src) for host '$host', but no tea login named '$canon' exists." >&2
             echo "       Refusing to borrow another login. Acting as a different identity would satisfy gate 16 mechanically while violating it." >&2
             echo "       Create it with:  ~/.config/mosaic/tools/fleet/seat-logins.sh --apply --seat $ident" >&2