tmux: add background task/error alerts and visual cues

[connorads]

Why

Long-running builds/tests can fail silently in background panes. Tmux supports hooks/options that can surface activity and command failures with minimal noise.

Scope

• Add alert workflow using tmux hooks/options (monitor-activity, command-error, pane-died as appropriate)
• Add visual cues in status/window styles for attention-needed panes
• Keep behaviour lightweight and non-spammy
• Ensure compatibility with current status scripts and mode indicator setup

Acceptance criteria

• Background pane activity/failures are visibly surfaced
• Alerts are actionable but not noisy
• Existing status bar behaviour remains readable on narrow widths
• Any new keybindings/options are documented in ~/.config/tmux/help.md