Add the Witch role

[JuneQQQ]

Summary

Implements the Witch role, requested in #2.

The Witch is a villager with two one-time potions. Each night she is told who
the werewolves attacked and may:

• spend a healing potion to save that victim, and/or
• spend a poison potion to kill any one living player.

Closes #2

Changes

• roles.py — new Role.WITCH; standard_setup() seats one once the village
  has five or more seats.
• engine.py — _witch_action() runs the Witch's turn right after the Doctor.
  The night phase is reworked: it now collects multiple simultaneous deaths
  (_announce_deaths) — the wolf kill and the Witch's poison — and each death
  can still trigger a Hunter shot. Potion use is tracked on GameState, so each
  potion is strictly once per game.
• agents/base.py — new concrete Agent.witch_turn(). RandomAgent and
  LLMAgent override it; the CLI's HumanAgent prompts for it. Existing agents
  need no change.
• prompts/templates.py — a dedicated witch_request() template; the offline
  mock recognises the witch decision kind and conservatively banks both
  potions.

Checklist

• ruff check . passes
• mypy deepwolf passes
• pytest passes (43 tests; 3 new for the Witch)
• Determinism preserved
• CHANGELOG.md and README.md updated

Notes for reviewers

The night phase changed shape — from "one victim" to "a list of deaths". The
healing potion and the Doctor's protection both cancel the wolf kill
independently. The Witch is briefed via a private WITCH_NIGHT_INFO event, so
the LLM agent sees the victim naturally in its prompt log.

[JuneQQQ]

Code review (post-merge record).

• The night-phase rework is the substantive change. _announce_deaths skips a target that an earlier death in the same night already removed (the if not s.player(pid).alive guard) — that correctly handles the case where a Hunter shot, fired by the first death, takes the Witch's poison target. ✅
• Potion state lives on GameState (witch_heal_used / witch_poison_used), and _witch_action flips the flag only when a potion is actually spent — so a once-per-game potion is genuinely once per game. ✅
• The Witch is briefed with a private WITCH_NIGHT_INFO event before her view is built, so the LLM agent sees the attacked player in its prompt log without any special-casing. Clean. ✅
• victim is not None is re-checked inside the heal branch, so the type stays sound and a heal can't fire without a victim. ✅
• CI green on 3.10–3.12.

Solid PR — the multi-death refactor was the right call rather than special-casing poison. Merged via squash.