Expand get_current_dir docs to explain when nil is returned (non-oil
buffer, non-files adapter, buffer context change). Add inline code
examples showing the capture-before-call pattern for Telescope keymaps.
Update the existing Telescope keymap example in oil.txt to guard against
nil. Add a dedicated Telescope recipe in recipes.md with find_files and
live_grep examples.
Add a defensive nil guard in posix_to_os_path for edge-case Windows
paths where the drive letter match fails.
Closes#682
