feat(s3): create bucket interactively during auth when unconfigured

Problem: when a user runs `:Pending s3 auth` with no bucket configured,
auth succeeds but offers no way to create the bucket. The user must
manually run `aws s3api create-bucket` and update their config.

Solution: add `util.input()` coroutine-aware prompt wrapper and a
`create_bucket()` flow in `s3.lua` that prompts for bucket name and
region, handles the `us-east-1` LocationConstraint quirk, and logs a
config snippet on success. Called automatically from `auth()` when
`sync.s3.bucket` is absent.

lua/pending/sync/util.lua

@@ -35,6 +35,21 @@ function M.system(args, opts)
   return coroutine.yield() --[[@as { code: integer, stdout: string, stderr: string }]]
 end
 
+---@param opts? {prompt?: string, default?: string}
+---@return string?
+function M.input(opts)
+  local co = coroutine.running()
+  if not co then
+    error('util.input() must be called inside a coroutine')
+  end
+  vim.ui.input(opts or {}, function(result)
+    vim.schedule(function()
+      coroutine.resume(co, result)
+    end)
+  end)
+  return coroutine.yield() --[[@as string?]]
+end
+
 ---@param name string
 ---@param fn fun(): nil
 function M.with_guard(name, fn)