Rename phase-numbered tests and drop SPEC pointer from stub messages

Rename the four commands_list_includes_phaseNN_entries tests after the
command namespace they cover (wiki, link health, export, diary), and
strip the "see SPEC §13.1" pointer from the not-yet-implemented runtime
notifications.

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

lua/nuwiki/commands.lua

@@ -477,7 +477,7 @@ end
 local function _not_yet(name)
   return function()
     vim.notify(
-      'nuwiki: ' .. name .. ' is not yet implemented — see SPEC §13.1',
+      'nuwiki: ' .. name .. ' is not yet implemented',
       vim.log.levels.WARN
     )
   end

lua/nuwiki/keymaps.lua

@@ -138,7 +138,7 @@ end
 local function deferred(name)
   return function()
     vim.notify(
-      'nuwiki: ' .. name .. ' is not yet implemented — see SPEC §13.1',
+      'nuwiki: ' .. name .. ' is not yet implemented',
       vim.log.levels.WARN
     )
   end