Add Pacman mini-game

[imMatheus]

Summary

• Adds a new Pacman mini-game (codex-rs/tui/src/games/pacman.rs) alongside the existing mini-games (Flappy Bird, Wordle, Snake, Tetris, Connect 4, Subway Surfer).
• 21x13 maze with regular pellets, four power pellets, and three ghosts that chase Pacman, flee when scared, and respawn after being eaten.
• Wires the new variant through MiniGameKind (core config), the game instantiation match, the config-persistence key in app.rs, and the game picker list in chatwidget.rs.

Gameplay

• Arrow keys or WASD to move.
• Eat all pellets to win; touching a non-scared ghost ends the run.
• Power pellets (◉ in the four corners) scare the ghosts for ~7s — run them over for 200 points each. They flash near the end of the timer and respawn at their home tile after being eaten.
• Direction-aware Pacman glyph (◐ / ◑ / ◒ / ◓) and per-ghost colors.

Test plan

• cargo check -p codex-tui passes
• cargo test -p codex-tui --lib games::pacman — all 5 unit tests pass (spawn, pellet consumption, wall blocking, ghost collision, power pellet scaring ghosts)
• cargo clippy -p codex-tui --lib --no-deps clean for the new file
• Launch the TUI, open the mini-game picker, select "Pacman", and verify rendering/input while the agent is running