Move development scripts into development/, leaving scripts/ plugin-only

The root scripts/ folder now holds only download_bin.vim, the one script
the plugin itself invokes (via :NuwikiInstall and the vim-plug/Dein build
hooks). All developer tooling — the start-nvim/start-vim launchers,
test-personal-wiki, the keymap harnesses, and syntax-diag — moves to
development/ alongside the developer docs.

REPO_ROOT resolution is unchanged (development/ sits one level under root,
same as scripts/ did); only the explicit cross-references, the CI harness
invocations, the README launcher paths, and the ONBOARDING structure tree
are updated.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -93,6 +93,6 @@ jobs:
           sudo apt-get install -y --no-install-recommends vim
           vim --version | head -1
       - name: run Neovim keymap harness
-        run: ./scripts/test-keymaps.sh
+        run: ./development/test-keymaps.sh
       - name: run Vim keymap harness
-        run: ./scripts/test-keymaps-vim.sh
+        run: ./development/test-keymaps-vim.sh