build: migrate test framework from plenary to busted

Problem: plenary.nvim is deprecated. The test suite depends on
plenary's async test runner and coroutine-based utilities, tying the
project to an unmaintained dependency. CI also tests against Neovim
0.8-0.11, which are no longer relevant.

Solution: replace plenary with busted + nlua (nvim -l). Convert all
async test patterns (a.wrap, a.util.sleep, a.util.scheduler) to
synchronous equivalents using vim.wait. Rename tests/ to spec/ to
follow busted convention. Replace the CI test matrix with
nvim-busted-action targeting stable/nightly only. Add .busted config,
luarocks test_dependencies, and update the nix devshell.

spec/test_util.lua

@@ -0,0 +1,176 @@
+local cache = require('oil.cache')
+local test_adapter = require('oil.adapters.test')
+local util = require('oil.util')
+local M = {}
+
+M.reset_editor = function()
+  require('oil').setup({
+    columms = {},
+    adapters = {
+      ['oil-test://'] = 'test',
+    },
+    prompt_save_on_select_new_entry = false,
+  })
+  vim.cmd.tabonly({ mods = { silent = true } })
+  for i, winid in ipairs(vim.api.nvim_tabpage_list_wins(0)) do
+    if i > 1 then
+      vim.api.nvim_win_close(winid, true)
+    end
+  end
+  vim.api.nvim_win_set_buf(0, vim.api.nvim_create_buf(false, true))
+  for _, bufnr in ipairs(vim.api.nvim_list_bufs()) do
+    vim.api.nvim_buf_delete(bufnr, { force = true })
+  end
+  cache.clear_everything()
+  test_adapter.test_clear()
+end
+
+local function throwiferr(err, ...)
+  if err then
+    error(err)
+  else
+    return ...
+  end
+end
+
+M.await = function(fn, nargs, ...)
+  local done = false
+  local results
+  local n_results = 0
+  local args = { ... }
+  args[nargs] = function(...)
+    results = { ... }
+    n_results = select('#', ...)
+    done = true
+  end
+  fn(unpack(args, 1, nargs))
+  vim.wait(10000, function()
+    return done
+  end, 10)
+  if not done then
+    error('M.await timed out')
+  end
+  return unpack(results, 1, n_results)
+end
+
+M.await_throwiferr = function(fn, nargs, ...)
+  return throwiferr(M.await(fn, nargs, ...))
+end
+
+M.oil_open = function(...)
+  M.await(require('oil').open, 3, ...)
+end
+
+M.wait_for_autocmd = function(autocmd)
+  local triggered = false
+  local opts = {
+    pattern = '*',
+    nested = true,
+    once = true,
+  }
+  if type(autocmd) == 'table' then
+    opts = vim.tbl_extend('force', opts, autocmd)
+    autocmd = autocmd[1]
+    opts[1] = nil
+  end
+  opts.callback = vim.schedule_wrap(function()
+    triggered = true
+  end)
+  vim.api.nvim_create_autocmd(autocmd, opts)
+  vim.wait(10000, function()
+    return triggered
+  end, 10)
+  if not triggered then
+    error('wait_for_autocmd timed out waiting for ' .. tostring(autocmd))
+  end
+end
+
+M.wait_oil_ready = function()
+  local ready = false
+  util.run_after_load(
+    0,
+    vim.schedule_wrap(function()
+      ready = true
+    end)
+  )
+  vim.wait(10000, function()
+    return ready
+  end, 10)
+  if not ready then
+    error('wait_oil_ready timed out')
+  end
+end
+
+---@param actions string[]
+---@param timestep integer
+M.feedkeys = function(actions, timestep)
+  timestep = timestep or 10
+  vim.wait(timestep)
+  for _, action in ipairs(actions) do
+    vim.wait(timestep)
+    local escaped = vim.api.nvim_replace_termcodes(action, true, false, true)
+    vim.api.nvim_feedkeys(escaped, 'm', true)
+  end
+  vim.wait(timestep)
+  vim.api.nvim_feedkeys('', 'x', true)
+  vim.wait(timestep)
+end
+
+M.actions = {
+  ---Open oil and wait for it to finish rendering
+  ---@param args string[]
+  open = function(args)
+    vim.schedule(function()
+      vim.cmd.Oil({ args = args })
+      if vim.b.oil_ready then
+        vim.api.nvim_exec_autocmds('User', {
+          pattern = 'OilEnter',
+          modeline = false,
+          data = { buf = vim.api.nvim_get_current_buf() },
+        })
+      end
+    end)
+    M.wait_for_autocmd({ 'User', pattern = 'OilEnter' })
+  end,
+
+  ---Save all changes and wait for operation to complete
+  save = function()
+    vim.schedule_wrap(require('oil').save)({ confirm = false })
+    M.wait_for_autocmd({ 'User', pattern = 'OilMutationComplete' })
+  end,
+
+  ---@param bufnr? integer
+  reload = function(bufnr)
+    M.await(require('oil.view').render_buffer_async, 3, bufnr or 0)
+  end,
+
+  ---Move cursor to a file or directory in an oil buffer
+  ---@param filename string
+  focus = function(filename)
+    local lines = vim.api.nvim_buf_get_lines(0, 0, -1, true)
+    local search = ' ' .. filename .. '$'
+    for i, line in ipairs(lines) do
+      if line:match(search) then
+        vim.api.nvim_win_set_cursor(0, { i, 0 })
+        return
+      end
+    end
+    error('Could not find file ' .. filename)
+  end,
+}
+
+---Get the raw list of filenames from an unmodified oil buffer
+---@param bufnr? integer
+---@return string[]
+M.parse_entries = function(bufnr)
+  bufnr = bufnr or 0
+  if vim.bo[bufnr].modified then
+    error("parse_entries doesn't work on a modified oil buffer")
+  end
+  local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, true)
+  return vim.tbl_map(function(line)
+    return line:match('^/%d+ +(.+)$')
+  end, lines)
+end
+
+return M