tmux: hooks support (set-hook)

[daiimus]

Summary

Support tmux hooks for custom event-driven behavior beyond the built-in control mode notifications.

What it is

tmux hooks allow registering commands to run when specific events occur. In control mode, hooks can be used to send additional information when events happen that aren't covered by the standard `%`-notifications.

Commands needed

Command	Purpose
`set-hook [-g] hook-name command`	Register a hook
`show-hooks [-g]`	List registered hooks

Why this matters

Hooks extend the notification system. For example:

• `after-select-pane` — custom action when pane focus changes
• `after-resize-pane` — custom action on resize
• `client-attached` / `client-detached` — custom actions on client lifecycle

This is mainly relevant if Geistty needs to react to events that don't have built-in `%`-notifications.

Priority

Low — the built-in notifications cover the important events. Hooks are for advanced customization.

Upstream potential

Fully upstreamable.

[daiimus]

Current Implementation

Swift side (Geistty)

• NOT FOUND — No set-hook implementation, no after-* / before-* tmux hook patterns in application code

Zig fork side (ios-external-backend)

• NOT FOUND — No hook support in the fork

What upstream needs

• set-hook command support in the viewer to register for tmux lifecycle events
• Hook notifications would provide fine-grained event callbacks (e.g., after-new-window, after-kill-pane, before-select-window)
• Could be used to trigger more efficient state updates instead of polling via list-windows after every notification
• Low priority: the current notification-based approach (%window-add, %layout-change, etc.) covers the most common cases

[daiimus]

Plain SSH vs tmux mode

tmux-only. Hooks are a tmux server-side event system. Plain SSH sessions have no equivalent lifecycle event subscription mechanism.

[daiimus]

Upstream Ghostty impact

Directly applicable but low value without consumers. Same protocol work (set-hook command + hook notification parsing). Hooks become valuable when there's advanced UI consuming the events — status bar updates (#93), session management (#91), or optimized state tracking. Until those features exist, hooks are a solution without a problem on both platforms.