[FEATURE] Worktree session names should show task content, not hostname-based branch names

Summary

When the Agent tool uses isolation: "worktree", sessions appear in the Remote Control mobile app with auto-generated git branch names as the session title.

Current behavior:
Session names appear as: {hostname}-{random-word}-{task}
Example: macmini26nomini-airport-staged-xxxxx

The hostname prefix is long and the random words (airport, precious, staged…) are meaningless — it's impossible to identify what each session is doing from the mobile app.

Expected behavior:
Session names should show the task description or first line of the prompt, not the git branch name.

Context

• Using Remote Control from iPhone to monitor Mac mini running multiple parallel worktree sessions
• With several worktree sessions running simultaneously, the session list becomes completely unreadable

Proposed Solution

Decouple the git branch name (needed for git uniqueness) from the displayed session name. Use the task prompt's first line or a --session-name parameter as the display name.