fix(init): guard read_file_lines against directory paths (#189)

Problem: when a commit contains a submodule or a symlink to a directory,
`read_file_lines` passes the path to `io.open` which succeeds, then
`f:lines()` raises `"Is a directory"` inside the decoration provider.

Solution: check `vim.fn.isdirectory(path)` before `io.open` and return
nil early, which `compute_hunk_context` already handles gracefully.

lua/diffs/init.lua

@@ -347,6 +347,9 @@ end
 ---@param path string
 ---@return string[]?
 local function read_file_lines(path)
+  if vim.fn.isdirectory(path) == 1 then
+    return nil
+  end
   local f = io.open(path, 'r')
   if not f then
     return nil