Always use pane ID as the runner index

[mvanderkamp]

With tmux list-panes -a we can get all panes across all windows and sessions, allowing us to precisely track our runner no matter where it is. But to do this we need to make g:VimuxRunnerIndex always be the pane ID.

This will be a breaking change for any Window runner users who have crafted custom configuration or scripts that modify g:VimuxRunnerIndex. I'm going to spend some time thinking about whether there's a way to keep supporting the old functionality, but I'd like to have this code sitting here for comment in the meantime.

[Copilot]

Pull Request Overview

This PR refactors Vimux to consistently use tmux pane IDs as runner identifiers instead of using different types (pane ID vs window ID) based on the runner type. This simplifies runner tracking by leveraging tmux list-panes -a to find panes across all windows and sessions.

• Removes the conditional logic that switched between pane IDs and window IDs based on runner type
• Introduces a new s:selectRunner() function that handles both window and pane selection uniformly
• Simplifies the s:hasRunner() function to only check for pane existence using list-panes -a

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