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]>

DESIGN.md

@@ -296,6 +296,27 @@ lines.
   The root can't branch out (it's the whole document), and neither can a
   portal (its document is one of its own already).
 
+**Export and import** (settled in part 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, and so on down to `######`; the tags as a list between
+  two `---` lines, then the body. `:export txt` writes an outline: a title
+  per line, two spaces a level, its tags after it, and its body under it a
+  level further in, each line after a `| ` (the user's pick).
+- The file goes next to the document and is named after it (`garden.md`),
+  or where `:export md file` says (`.md` or `.txt` is added). As vim's `:w`
+  to another file does, it leaves a file that's there alone unless it's
+  `:export!` (the user's pick).
+- Typed in visual mode, `:export` writes just the selection (the user's
+  pick), named after its first branch (`raised-beds.md`): with `v` exactly
+  the nodes chosen, with `V` their branches.
+- `:import file` reads Markdown headings, a bulleted list or indented lines
+  (what both exports write, and what the clipboard reads), and the nodes go
+  under the cursor's node as its last children, laid out as a mind map, as
+  one undo step (the user's pick). `#tags` at the end of a line are its
+  tags, and lines after a `| ` are the body of the node above. Looks and
+  cross-links stay out of both exports.
+
 **Views, panes and popups** (settled in stage 7)
 - The Note view is made of parts: the title, the tags, the body, each link
   (out, then in) and each child. `j` and `k` step through them, with a bar
@@ -718,7 +739,8 @@ cumulus/
    │               (keys.toml's template), theme_file (themes laid over the built-in one)
    ├─ storage/     document_file (RON), write_atomically, names (file names from titles), link_paths
    │               (portal paths: relative, with /), backup, lock, stamp (noticing changes made
-   │               outside), memory (memory.ron), markdown (nodes as Markdown, for the clipboard)
+   │               outside), memory (memory.ron), markdown (nodes as Markdown, for the clipboard
+   │               and :export; text read back as nodes), plain_text (:export txt)
    └─ app/         mod.rs (App), input (keys and the mouse, routed), update (carrying actions out),
                    render, terminal (the main loop, setup, teardown, panic hook), startup (what
                    `cumulus` opens), documents (the open documents and their files), files
@@ -730,12 +752,13 @@ cumulus/
                    search (`/`), jumps (each pane's jump list), marks (capital marks, `:marks`),
                    connect (connect mode), portals (what portals show, going through them,
                    `:portal`, backlinks, renames), branch_out (`:branchout`), panes (the split
-                   tree), popup (help, menu, pickers, card, questions)
+                   tree), export (:export, :import), popup (help, menu, pickers, card, questions)
 ```
 
 The end-to-end tests are in `app/tests.rs` (with `app/tests/files.rs`,
 `app/tests/config.rs`, `app/tests/vim.rs`, `app/tests/connect.rs`,
-`app/tests/portals.rs`, `app/tests/arrange.rs` and `app/tests/branch_out.rs`):
+`app/tests/portals.rs`, `app/tests/arrange.rs`, `app/tests/branch_out.rs` and
+`app/tests/export.rs`):
 they copy the sample
 documents to a temporary folder, type keys and click on a test terminal, and
 check the screen, the documents and the files.