tmux: catppuccin status line + native prefix module (tpack)

[returnsvoidjanet]

Ports the full catppuccin-themed tmux status line from the shared lab config into these dotfiles.

Changes to tmux/tmux.conf:

• catppuccin (mocha) theme + rounded window status.
• Native "prefix" status module — shows the prefix-key state (mauve = prefix, yellow = copy-mode, red = pane-sync, dim when idle), connected/flowing with the other segments. Built as a real catppuccin custom module (defined before catppuccin loads, per the custom-status tutorial) rather than the tmux-prefix-highlight plugin, so it connects into the bar instead of floating as a pill.
• Status modules: prefix · application · cpu · ram · session · uptime · battery.
• QoL: C-r reload, x=kill-pane, |/_ splits keeping cwd, inactive-pane dimming, activity alerts.
• tpack instead of tpm for plugin management.
• opensessions off by default (kept your # LLMs are opt-in intent — commented out).

First load needs plugins installed: run tpack once, then reload so catppuccin's utils/status_module.conf (which the prefix module sources) exists.