Share no ssh connection when the socket path would be too long

macOS accepts socket paths of at most 103 characters. Count the folder,
the file name, the 40-character hash and the temporary end ssh adds; when
the total is over 103, leave the connection-sharing options out. Find,
make and check the socket folder once per run instead of for every
command.

docs/superpowers/specs/2026-10-15-remote-repositories-over-ssh-design.md

@@ -69,7 +69,12 @@ A path on the server that is `~` or starts with `~/` is handed to the server's s
     `gites` folder in the user's cache folder, made so that only the user can open it;
     never in `/tmp`. If neither folder can be found or made, these three options are
     left out and every command opens a new connection. `%C` is a hash ssh makes from
-    the host, port and user, so the path stays short.
+    the host, port and user, 40 characters long. macOS accepts a socket path of at most
+    103 characters (Linux 107), and while ssh sets up a shared connection it adds a
+    temporary end of up to 18 characters to the file name. So gites counts the folder's
+    length + 1 + 6 (`gites-`) + 40 + 18; if that is more than 103 (one limit on every
+    system), these three options are left out too. The folder is found, made and checked
+    once when gites first runs ssh, not for every command.
   - On Windows, ssh cannot share connections, so these three options are left out and
     every command opens a new connection.
 - Quoting: every word of the git command is put in single quotes, and a `'` inside a