Support worktree sessions on existing branches, including worktree reuse

Support worktree sessions on existing branches, including worktree reuse

When creating a worktree session, I want to work directly on the branch I select—not automatically create a new branch based on it.

Current behavior: Selecting features/8887 creates a worktree on a generated branch such as louibof-literate-carnival. The selected branch is treated as a base branch, which is unexpected when continuing existing work.

Requested behavior:

• Offer an explicit choice between use an existing branch and create a new branch from it.

• When using an existing branch, check out that exact branch in the worktree.

• If a worktree already exists for the branch, offer to reuse it when safe.

• If reuse is impossible, explain why instead of creating a replacement branch.

• Remember the preferred mode for future sessions.

Why it matters: Worktree isolation is useful for continuing existing feature branches and PRs, not just starting new work. Automatically creating another branch adds confusion and unnecessary branch management.

Start by locating the worktree session creation flow and branch-selection UI described in the issue. Compare the existing-branch and new-branch paths, then verify branch reuse, clear handling when reuse is impossible, and preference persistence on Windows.