Add configurable worktree branch prefix to CLI (parity with Desktop)

Feature Request

Claude Code Desktop allows users to configure a branch prefix that gets prepended to every worktree branch name (Settings → "Worktree location"). This is useful for organizing Claude-created branches by user or team.

The CLI has no equivalent setting — worktrees always use worktree-<name>.

Desired Behavior

A settings.json option (e.g. worktreeBranchPrefix) that lets CLI users configure a prefix:

{
  "worktreeBranchPrefix": "claude/"
}

So claude --worktree fix-auth would create a branch named claude/fix-auth instead of worktree-fix-auth.

Motivation

• Desktop/CLI parity: Desktop already supports this; CLI users should have it too
• Branch organization: Teams with naming conventions (e.g. user/branch, claude/branch) benefit from prefixing
• Related: #28761 requests removing the hardcoded worktree- prefix — a configurable prefix would address that use case as well