feat(reversi): classic Reversi / Othello with hotseat and AI

Summary

• Adds src/reversi/: classic Reversi / Othello on an 8×8 board with documented rules (opening centre four, eight-ray flips, pass, majority / draw).
• Hotseat two-player and vs-computer modes (heuristic AI), toggleable legal-move hints, original visuals.
• Vitest: 13 passed (npm test). Amplify + customHttp.yml registered; root Games README intentionally not updated.

Test plan

• cd src/reversi && npm ci && npm test && npm run build
• Opening: Black has four legal dots; first move flips one White disc
• Vs computer: White replies automatically; game reaches a win/draw
• Two players: alternate moves on one device; H toggles hints; N resets