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/path_spec.lua

@@ -0,0 +1,32 @@
+local pathutil = require('oil.pathutil')
+describe('pathutil', function()
+  it('calculates parent path', function()
+    local cases = {
+      { '/foo/bar', '/foo/' },
+      { '/foo/bar/', '/foo/' },
+      { '/', '/' },
+      { '', '' },
+      { 'foo/bar/', 'foo/' },
+      { 'foo', '' },
+    }
+    for _, case in ipairs(cases) do
+      local input, expected = unpack(case)
+      local output = pathutil.parent(input)
+      assert.equals(expected, output, string.format('Parent path "%s" failed', input))
+    end
+  end)
+
+  it('calculates basename', function()
+    local cases = {
+      { '/foo/bar', 'bar' },
+      { '/foo/bar/', 'bar' },
+      { '/', nil },
+      { '', nil },
+    }
+    for _, case in ipairs(cases) do
+      local input, expected = unpack(case)
+      local output = pathutil.basename(input)
+      assert.equals(expected, output, string.format('Basename "%s" failed', input))
+    end
+  end)
+end)