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.

Makefile

@@ -1,44 +0,0 @@
-## help: print this help message
-.PHONY: help
-help:
-	@echo 'Usage:'
-	@sed -n 's/^##//p' ${MAKEFILE_LIST} | column -t -s ':' |  sed -e 's/^/ /'
-
-## all: lint and run tests
-.PHONY: all
-all: lint test
-
-## test: run tests
-.PHONY: test
-test:
-	luarocks test --local
-
-## lint: run selene and stylua
-.PHONY: lint
-lint:
-	selene --display-style quiet .
-	stylua --check lua spec
-
-## profile: use LuaJIT profiler to profile the plugin
-.PHONY: profile
-profile: scripts/benchmark.nvim
-	nvim --clean -u perf/bootstrap.lua -c 'lua jit_profile()'
-
-## flame_profile: create a trace in the chrome profiler format
-.PHONY: flame_profile
-flame_profile: scripts/benchmark.nvim
-	nvim --clean -u perf/bootstrap.lua -c 'lua flame_profile()'
-
-## benchmark: benchmark performance opening directory with many files
-.PHONY: benchmark
-benchmark: scripts/benchmark.nvim
-	nvim --clean -u perf/bootstrap.lua -c 'lua benchmark()'
-	@cat perf/tmp/benchmark.txt
-
-scripts/benchmark.nvim:
-	git clone https://github.com/stevearc/benchmark.nvim scripts/benchmark.nvim
-
-## clean: reset the repository to a clean state
-.PHONY: clean
-clean:
-	rm -rf perf/tmp profile.json