Support ticket-led agent display names

[nathanestone-alt]

We use the pane label and tab title for a stable seat identity, while the agent display name carries the active work identity (ticket plus arc). A natural work slug is 1022-fix3b-recolor, but Herdr currently rejects it because agent names must begin with a lowercase letter. The required issue- prefix adds no information and makes the volatile name harder to scan.

Reproduction on Herdr 0.8.2:

herdr agent rename <pane-id> 1022-fix3b-recolor

Result: invalid_agent_name.

Would Herdr consider supporting digit-led agent names, or providing a writable pane subtitle intended for this kind of volatile work label? Either would let the stable seat name and active ticket/arc remain visually distinct.