Export to Markdown or text, and import text as nodes (stage 9c)

:export md writes the document as Markdown headings in the clipboard's
format (the user's pick): # for the root and each loose node, ## for
their children, down to ######, the tags between --- lines, then the
body. :export txt writes an indented outline, tags after each title and
the body under it after "| " (the user's pick), from the new
storage::plain_text. The file goes next to the document, named after it,
or where it's told; as vim's :w to another file does, an existing file
needs :export! (the user's pick). Typed in visual mode, :export writes
just the selection, named after its first branch (the user's pick).

:import file reads Markdown headings, a list or indented lines with the
clipboard's reader, now also taking #tags at the end of a line and "| "
body lines, so a text export reads back whole. The nodes go under the
cursor's node, laid out as a mind map, as one undo step (the user's
pick).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/mod.rs

@@ -27,6 +27,7 @@ pub mod commands;
 mod configure;
 pub mod connect;
 pub mod documents;
+mod export;
 mod files;
 mod input;
 pub mod jumps;