phase 0: scaffold workspace, plugin layout, and CI

Lay down the empty repo skeleton defined in SPEC.md §5 so subsequent
phases have somewhere to land:

- Cargo workspace (resolver v2, edition 2021, MSRV 1.83) with
  nuwiki-core, nuwiki-lsp, nuwiki-ls — dep direction matches §6.2.
- Vim/Neovim plugin directory layout (plugin, lua, autoload, ftdetect,
  ftplugin, syntax, doc, scripts) with header-only stubs.
- .gitea/workflows/ci.yaml running fmt, clippy -D warnings, and tests
  pinned to Rust 1.83.
- README, dual MIT/Apache-2.0 license texts, .gitignore.

Verified: cargo check / fmt --check / clippy / test all clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugin/nuwiki.vim

@@ -0,0 +1,8 @@
+" plugin/nuwiki.vim — universal entry point for Vim and Neovim.
+" Loaded once per session by all plugin managers.
+" Implementation deferred to Phase 9 (Editor Glue).
+
+if exists('g:loaded_nuwiki')
+  finish
+endif
+let g:loaded_nuwiki = 1