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).

scrapers/atcoder.py

@@ -606,6 +606,8 @@ class AtcoderScraper(BaseScraper):
                 contest_id=contest_id,
                 problems=problems,
                 url=f"https://atcoder.jp/contests/{contest_id}/tasks/{contest_id}_%s",
+                contest_url=f"https://atcoder.jp/contests/{contest_id}",
+                standings_url=f"https://atcoder.jp/contests/{contest_id}/standings",
             )
         except Exception as e:
             return self._metadata_error(str(e))