fish: clear existing completions before defining aliases (#1165)

Fish ships with built-in completions for common commands like `j` (share/completions/j.fish for autojump). These override zoxide's completions when using `--cmd j`. Clear them before defining aliases.

templates/fish.txt

@@ -117,9 +117,11 @@ end
 {%- when Some with (cmd) %}
 
 abbr --erase {{cmd}} &>/dev/null
+complete --erase --command {{cmd}}
 alias {{cmd}}=__zoxide_z
 
 abbr --erase {{cmd}}i &>/dev/null
+complete --erase --command {{cmd}}i
 alias {{cmd}}i=__zoxide_zi
 
 {%- when None %}