refactor: standardize log prefixes to [diffs.nvim]:

Problem: `lib.lua` used `[diffs]` and `debug.lua`/`init.lua` used
`[diffs.nvim]` (no colon) while the rest of the codebase used
`[diffs.nvim]:`.

Solution: Normalize all `vim.notify` prefixes to `[diffs.nvim]:` for
consistency with `log.lua` and the other modules.

lua/diffs/debug.lua

@@ -63,7 +63,7 @@ function M.dump()
   if f then
     f:write(vim.json.encode(result))
     f:close()
-    vim.notify('[diffs.nvim] debug dump: ' .. path, vim.log.levels.INFO)
+    vim.notify('[diffs.nvim]: debug dump: ' .. path, vim.log.levels.INFO)
   end
 end