fix(test): resolve busted migration test isolation issues (#25)

* fix(test): resolve busted migration test isolation issues

Problem: Two issues introduced during the plenary-to-busted migration
(6be0148). First, altbuf_spec waited for two BufEnter events but
oil:// → file resolution only fires one async BufEnter (the synchronous
one from vim.cmd.edit fires before wait_for_autocmd is registered).
Second, reset_editor kept the first window which could be a preview
window with stale oil_preview/oil_source_win state, causing
close_preview_window_if_not_in_oil to close the wrong window in
subsequent tests.

Solution: Wait for a single BufEnter in the altbuf test. Replace the
window-keeping logic in reset_editor with vim.cmd.new() +
vim.cmd.only() to guarantee a fresh window with no inherited state.
This also fixes the preview_spec.lua:30 timeout which had the same
root cause. 114/114 tests pass.

* fix(ci): fix nightly tests and remove unused Makefile

Problem: Neovim nightly's ftplugin/markdown.lua now calls
vim.treesitter.start() unconditionally, which crashes in CI where the
markdown parser is not installed. The Makefile is unused — CI runs
selene, stylua, and nvim-busted-action directly.

Solution: Change filetype plugin indent on to filetype on in
minimal_init.lua. Tests only need filetype detection for
vim.bo.filetype assertions, not ftplugin or indent loading. Remove
the Makefile.

spec/minimal_init.lua

@@ -2,7 +2,7 @@ vim.cmd([[set runtimepath=$VIMRUNTIME]])
 vim.opt.runtimepath:append('.')
 vim.opt.packpath = {}
 vim.o.swapfile = false
-vim.cmd('filetype plugin indent on')
+vim.cmd('filetype on')
 vim.fn.mkdir(vim.fn.stdpath('cache'), 'p')
 vim.fn.mkdir(vim.fn.stdpath('data'), 'p')
 vim.fn.mkdir(vim.fn.stdpath('state'), 'p')