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

lib/core/src/app_keys.h

@@ -326,7 +326,7 @@ inline constexpr KeyHelp kNotesName[] = {
 // shell: Shell
 inline constexpr KeyHelp kShell[] = {
     {"Enter", "run the line"},
-    {"Tab", "complete the command"},
+    {"Tab", "complete: a command, a path"},
     {"Fn ; .", "lines you typed before"},
     {"Alt ; .", "scroll back, forward"},
     {"Ctrl b", "your replies only, or all"},