Problem: opening oil as a float requires users to remap every entry
point (`-`, `:Oil`, startup autocmd) individually. There is no single
flag to make float the default everywhere.
Solution: add `default_to_float = false` to config. When `true`,
`M.open()` delegates to `M.open_float()` (covers `:Oil` and all keymap
invocations), and a `VimEnter` hook replaces the initial directory
buffer with a float when starting Neovim on a directory (e.g. `nvim .`).
No recursion risk — `open_float()` calls `vim.cmd.edit()` directly and
never goes through `M.open()`.
Based on: stevearc/oil.nvim#298
3fede7d809