agent_ui: Fix worktree naming regression when using existing branches

[rtfeldman]

When creating a new git worktree from an existing branch (e.g. main), the worktree directory was incorrectly named after the branch instead of getting a random adjective-noun name. Additionally, if a directory with that name already existed, the rollback logic could destructively delete the pre-existing worktree.

Changes:

• Generate a random adjective-noun name for worktree directories when no explicit name is provided
• Collect existing worktree directory names to avoid collisions
• Check generated paths against known worktree paths before creating
• Reduce random name retry count from 100 to 10
• Add regression test

Release Notes:

• Fixed a regression where creating a git worktree from an existing branch would name the worktree directory after the branch (instead of generating a random name)