feat(commands): implement :CP open [problem|contest|standings]

Problem: There was no way to open a problem or contest page in the
browser from within the plugin; users had to manually navigate to
the platform URL.

Solution: Add `contest_url` and `standings_url` to `MetadataResult`
and persist them via `cache.set_contest_data`. Add `cache.get_open_urls`
to assemble all three URLs from cache. Wire up `:CP open` in
`commands/init.lua` to call `vim.ui.open` on the resolved URL,
warning when none is available (e.g. CSES standings).

lua/cp/constants.lua

@@ -13,6 +13,7 @@ M.ACTIONS = {
   'race',
   'stress',
   'submit',
+  'open',
 }
 
 M.PLATFORM_DISPLAY_NAMES = {