feat(config): add default_to_float option (#173)

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

doc/oil.txt

@@ -61,6 +61,9 @@ The full list of options with their defaults:
       -- Oil will take over directory buffers (e.g. `vim .` or `:e src/`)
       -- Set to false if you want some other plugin (e.g. netrw) to open when you edit directories.
       default_file_explorer = true,
+      -- When true, oil always opens in a floating window. Applies to :Oil, all open() calls,
+      -- and directory buffers opened on startup (e.g. `nvim .`).
+      default_to_float = false,
       -- Id is automatically added at the beginning, and name at the end
       -- See :help oil-columns
       columns = {
@@ -293,6 +296,13 @@ The full list of options with their defaults:
 OPTIONS                                                              *oil-options*
 
 
+default_to_float                                          *oil.default_to_float*
+    type: `boolean` default: `false`
+    When `true`, oil always opens in a floating window. Applies to all
+    `open()` calls (`:Oil`, default keymaps) and to directory buffers opened
+    on startup (e.g. `nvim .`). Individual calls to `open_float()` or
+    `open()` still work regardless of this setting.
+
 skip_confirm_for_simple_edits                  *oil.skip_confirm_for_simple_edits*
     type: `boolean` default: `false`
     Before performing filesystem operations, Oil displays a confirmation popup to ensure