feat(keymaps): configurable map_prefix for the wiki command family (P2)

Mirror vimwiki's g:vimwiki_map_prefix. The whole <Leader>w* family was
hardcoded across the map layer; it is now built from a configurable prefix:

- Neovim: `map_prefix` setup() option (default '<Leader>w'), read by
  lua/nuwiki/keymaps.lua (buffer-local) and lua/nuwiki/init.lua (global
  entry points).
- Vim: g:nuwiki_map_prefix, applied via :exe in plugin/nuwiki.vim (global)
  and ftplugin/vimwiki.vim (buffer-local).

Setting a custom prefix relocates <prefix>{w,t,s,i,n,d,r,c,h,hh,ha} and
<prefix><Leader>{w,y,t,m,i}, leaving nothing under the old <Leader>w*.
Non-prefix maps (<CR>, gl*, headers, …) are untouched.

Tests: new test-keymaps-vim-prefix.vim harness (21 cases, wired into
test-keymaps-vim.sh) + map_prefix.relocates_wiki_family in test-keymaps.lua.
Docs: README, doc/nuwiki.txt, lua config comment, vimwiki-gap.md ticked.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

development/tests/test-keymaps.lua

@@ -820,6 +820,36 @@ vim.defer_fn(function()
     end
   end)
 
+  -- ===== Configurable map_prefix (vimwiki g:vimwiki_map_prefix) =====
+  -- Overriding map_prefix must relocate the whole <prefix>* family. We attach
+  -- to a throwaway scratch buffer with a custom prefix and assert the new
+  -- bindings exist there while the default <Leader>w* ones do not.
+  tobj_case('map_prefix.relocates_wiki_family', function()
+    local config = require('nuwiki.config')
+    local keymaps = require('nuwiki.keymaps')
+    local saved = config.options.map_prefix
+    config.options.map_prefix = '<Leader>n'
+    local buf = vim.api.nvim_create_buf(false, true)
+    vim.api.nvim_set_current_buf(buf)
+    local ok, err = pcall(function()
+      keymaps.attach(buf, { enabled = true, wiki_prefix = true, links = true, html_export = true })
+      local function bmap(lhs)
+        local d = vim.fn.maparg(lhs, 'n', false, true)
+        return not vim.tbl_isempty(d) and d.buffer == 1
+      end
+      for _, lhs in ipairs({
+        '<Leader>nw', '<Leader>nt', '<Leader>n<Leader>w', '<Leader>nr', '<Leader>nh', '<Leader>nha',
+      }) do
+        if not bmap(lhs) then error('relocated map missing: ' .. lhs) end
+      end
+      if bmap('<Leader>ww') then error('<Leader>ww still bound under custom prefix') end
+    end)
+    -- Always restore global state so later cases see the default prefix.
+    config.options.map_prefix = saved
+    vim.api.nvim_buf_delete(buf, { force = true })
+    if not ok then error(err) end
+  end)
+
   -- ===== Wiki picker (config-driven, no LSP) =====
   -- The picker must build its list from config so it works from any buffer
   -- before the server attaches.