Shell: Tab completes a path on the SD card (#67)

Past the command's name, Tab completes the word being typed as a path: a
folder keeps its slash to go on from, a file completed whole gets a space,
several candidates are listed. Any case typed, the name's own is taken.
After a file command the first slash is understood (`cat no` is /no).
The folder is read on the storage task, bounded to 400 entries looked at
and 24 candidates.

489 host tests (2 new). Checked on the device: a folder, a file inside it,
several candidates, no slash, another case, nothing matching.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

site/content/guide/shell.md

@@ -23,7 +23,7 @@ Type a command and press <kbd>Enter</kbd>. `help` lists them all; the [command r
 | `update check` | Whether a newer release exists |
 | `lora status` | What the radio is set to and what it has heard |
 
-- <kbd>Tab</kbd> **completes** the command's name. If several fit, it lists them.
+- <kbd>Tab</kbd> **completes** what you are typing: the command's name first, then **a path on the SD card**. `ls /no` and Tab gives `ls /notes/`; Tab again goes on inside the folder. If several names fit, it completes as far as they agree and lists them. You can type the name in any case, and after a file command you can leave out the first slash: `cat no` and Tab gives `cat /notes/`. A name with a space in it isn't completed.
 - <kbd>Fn</kbd> with up and down brings back **lines you typed before**.
 - <kbd>Alt</kbd> with up and down **scrolls back** through what was printed.
 - `clear` empties the screen, and `quit` (or Back) leaves.