Add lazyjira command to Custom Commands Compendium

I'd like to add this to the bottom of the [Custom Commands Compendium](https://github.com/jesseduffield/lazygit/wiki/Custom-Commands-Compendium), but the wiki is not writable by my account.

## Open lazyjira in a tmux popup

Launch [lazyjira](https://github.com/textfuel/lazyjira) from lazygit without switching terminal tabs. If inside tmux, it opens in a popup; otherwise it runs in lazygit's terminal output mode.

```yaml
customCommands:
- key: "J"
context: "global"
description: "Open lazyjira"
command: 'if [ -n "$TMUX" ]; then tmux popup -w 90% -h 90% -E lazyjira; else lazyjira; fi'
output: terminal
```

The target is the Custom Commands Compendium wiki; start by reviewing its existing entries and the supplied YAML snippet. Add the lazyjira/tmux popup command at the bottom, preserving the documented format. Done means the example appears in the compendium with its description and behavior intact.

Written by the indexing model from the issue text.