refactor: replace :CP login with :CP credentials subcommand

Problem: :CP login was a poor API — no way to clear credentials without
raw Lua, and the single command didn't scale to multiple operations.

Solution: replace login with a :CP credentials subcommand following the
same pattern as :CP cache. :CP credentials set [platform] prompts and
saves; :CP credentials clear [platform] removes one or all platforms.
Add cache.clear_credentials(), rename login.lua to credentials.lua,
update parse/dispatch/tab-complete, and rewrite vimdoc accordingly.

lua/cp/cache.lua

@@ -388,6 +388,18 @@ function M.set_credentials(platform, creds)
   M.save()
 end
 
+---@param platform string?
+function M.clear_credentials(platform)
+  if platform then
+    if cache_data._credentials then
+      cache_data._credentials[platform] = nil
+    end
+  else
+    cache_data._credentials = nil
+  end
+  M.save()
+end
+
 function M.clear_all()
   local creds = cache_data._credentials
   cache_data = {}