Track upstream support for numbered Herdr tabs

[arecarn]

Herdr does not currently display positional tab numbers independently of tab names. This makes the configured switch_tab = "prefix+1..9" binding harder to use because the target number must be inferred by counting tabs.

Track upstream discussion herdrdev/herdr#3299, which proposes a configurable tab_format with separate {index} and {name} placeholders.

Desired configuration:

[ui]
tab_format = " {index}:{name} "

Once upstream support is released:

• update Herdr
• configure numbered tab labels in herdr/.config/herdr/config.toml
• verify numbering follows tab position when tabs are moved or closed
• verify custom tab names remain independent of numbering