Add the arena leaderboard

[JuneQQQ]

Summary

Implements the arena leaderboard requested in #4.

A leaderboard ranks agents fairly. Each competitor is measured under
identical, seeded conditions:

• it plays the werewolves against a fixed reference village, and
• it plays the village against fixed reference werewolves.

Its score is the mean of the two win rates. Because both match-ups reuse the
same seeds for every competitor, the ranking is reproducible and the only
variable is the agent itself.

Closes #4

Changes

• New deepwolf/arena/leaderboard.py — Leaderboard, LeaderboardReport
  (with render() and to_markdown()), LeaderboardEntry. It reuses the
  existing Arena for each match-up rather than duplicating the game loop.
• New deepwolf leaderboard CLI command, with --markdown PATH to export a
  paste-ready table and --provider env / --model to add a real model as a
  competitor.
• 7 new tests (ranking, reproducibility, Markdown, score arithmetic, progress).

Checklist

• ruff check . passes
• mypy deepwolf passes
• pytest passes (50 tests; 7 new)
• CHANGELOG.md and README.md updated

Notes for reviewers

A "competitor" is a faction-agnostic AgentBuilder (Callable[[int], Agent]),
so the same agent can be slotted into either side. Leaderboard._arena composes
two builders into the (player_id, role) -> Agent factory the Arena expects.

[JuneQQQ]

Code review (post-merge record).

• Leaderboard reuses Arena rather than duplicating the game loop — both match-ups are just two Arena runs with composed factories. Good, no logic drift risk. ✅
• Fairness holds: every competitor's two arenas use the same base_seed, so competitors face identical games and the score difference is purely the agent. ✅
• _arena composes two faction-agnostic AgentBuilders into the (player_id, role) -> Agent factory — clean separation, and it means a real model added via --provider env drops in with no special handling. ✅
• to_markdown() produces a paste-ready table; reproducibility is covered by a test. ✅
• Empty competitor field is rejected early with a clear ValueError. ✅
• CI green on 3.10–3.12.

Nice — this is what turns the arena into an actual benchmark. Merged via squash.