fix(diff): cast tonumber result to integer

Problem: LuaLS infers priority as integer from the = 0 initialiser
but tonumber returns number?, causing a cast-local-type diagnostic.

Solution: inline --[[@as integer]] cast after the tonumber call.

lua/pending/diff.lua

@@ -37,7 +37,7 @@ function M.parse_buffer(lines)
       local prio_str = stripped:match('^%[(%d+)%] ')
       local priority = 0
       if prio_str then
-        priority = tonumber(prio_str)
+        priority = tonumber(prio_str) --[[@as integer]]
         stripped = stripped:sub(#prio_str + 4)
       end
       local description, metadata = parse.body(stripped)