Allow naming/renaming background agents in `claude agents` view

Feature Request

Problem: Background agents in claude agents get auto-generated titles based on conversation content. There's no way to manually name or rename them, making it harder to identify agents at a glance when running several in parallel.

Proposed Solution: Allow users to assign a custom name/title to background agents, either:

1. At launch time (e.g., a flag when starting a background agent)
2. After the fact (e.g., a rename command from the claude agents view)

Current Workaround: None — you have to rely on the auto-generated title from the classifier.

Context: claude --name works for naming CLI sessions visible in /jobs, but doesn't apply to the claude agents view where background agents are listed.