chore: remove tmux after Zellij migration

[shmileee]

Summary

• stop managing and provisioning tmux
• remove the Homebrew package and tmux state during the tagged Ansible cleanup
• remove fallback documentation now that Zellij owns the terminal workflow

Stack

• depends on feat: migrate terminal workflow to Zellij #53
• merge only after the Zellij migration is accepted

Validation

• pre-commit run --all-files
• ansible-playbook --syntax-check main.yaml
• ansible-playbook --check --tags tmux main.yaml
• scoped chezmoi render, diff, dry-run, and managed-target checks
• rendered Zellij config passes zellij setup --check
• rendered config launched successfully in an isolated Alacritty/Zellij session

No local tmux files or running tmux processes were removed during validation.