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/s3.lua

@@ -66,6 +66,49 @@ local function ensure_sync_id(task)
   return sync_id
 end
 
+local function create_bucket()
+  local name = util.input({ prompt = 'S3 bucket name: ', default = 'pending.nvim' })
+  if not name or name == '' then
+    log.info('s3: bucket creation cancelled')
+    return
+  end
+
+  local region_cmd = base_cmd()
+  vim.list_extend(region_cmd, { 'configure', 'get', 'region' })
+  local region_result = util.system(region_cmd, { text = true })
+  local default_region = 'us-east-1'
+  if region_result.code == 0 and region_result.stdout then
+    local detected = vim.trim(region_result.stdout)
+    if detected ~= '' then
+      default_region = detected
+    end
+  end
+
+  local region = util.input({ prompt = 'AWS region: ', default = default_region })
+  if not region or region == '' then
+    region = 'us-east-1'
+  end
+
+  local cmd = base_cmd()
+  vim.list_extend(cmd, { 's3api', 'create-bucket', '--bucket', name, '--region', region })
+  if region ~= 'us-east-1' then
+    vim.list_extend(cmd, { '--create-bucket-configuration', 'LocationConstraint=' .. region })
+  end
+
+  local result = util.system(cmd, { text = true })
+  if result.code == 0 then
+    log.info(
+      's3: bucket created. Add to your config:\n  sync = { s3 = { bucket = "'
+        .. name
+        .. '", region = "'
+        .. region
+        .. '" } }'
+    )
+  else
+    log.error('s3: bucket creation failed — ' .. (result.stderr or 'unknown error'))
+  end
+end
+
 ---@param args? string
 ---@return nil
 function M.auth(args)
@@ -96,6 +139,10 @@ function M.auth(args)
       else
         log.info('s3: credentials valid')
       end
+      local s3cfg = get_config()
+      if not s3cfg or not s3cfg.bucket then
+        create_bucket()
+      end
     else
       local stderr = result.stderr or ''
       if stderr:find('SSO') or stderr:find('sso') then

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)