[Feature] Middle-truncate agent names in assignee / agent dropdowns

[HCH-hash]

Summary

Prevent agent names that share a long common prefix from becoming indistinguishable in dropdowns by middle-truncating (or showing handle + tooltip).

Problem

The assignee/agent dropdown truncates from the right. Agents whose names share a long prefix (e.g. backend-refactor-agent-v2, backend-refactor-agent-v3) become visually identical once truncated, making selection error-prone.

Proposed Direction

• Middle-truncate long labels (keep start and end visible), or
• Show the short handle prominently and put the full name in a title tooltip (or both).
• Apply consistently anywhere agent/assignee names are shown in constrained width.

Scope

• Product behavior
• Frontend
• Documentation (if design-system guidance is updated)
• Other

Additional Context

• Small UI change with outsized clarity benefit for teams that run many similarly named agents.
• No backend changes required.

[pullfrog]

  ^{｜ Build this ➔ ｜ Make a plan ➔}

[pikann]

Hi @HCH-hash , thank you so much for taking the time to open all these issues!
Really appreciate your dedication to the project. 🚀
I'll review and address them as soon as possible!