wt should reuse matching local branches and track matching remote branches when creating a worktree

[ukanga]

Problem

When creating a worktree from a name that already exists as a branch, wt should not always create a fresh branch from the default base.

Expected behavior:

• If name matches a local branch, check out that branch in the new worktree.
• If name matches a remote-tracking branch, create the worktree from that remote branch and track it.
• If multiple remote branches match, prompt the user to choose which remote to use.

Repro

1. Have a repo with origin/feature/remote-worktree-match.
2. Run wt session add feature/remote-worktree-match or wt new feature/remote-worktree-match.
3. Observe whether wt uses the matching branch/ref or falls back to the base branch.

Expected

wt should create the worktree on feature/remote-worktree-match when it exists locally, or on the matching remote branch when only origin/feature/remote-worktree-match exists.

Actual

Before the fix, wt could create a new branch from the base branch instead of using the existing branch/ref.

Acceptance Criteria

• Local branch match reuses the existing branch.
• Remote branch match checks out a tracked branch from that remote ref.
• Ambiguous remote matches prompt the user to choose.
• Tests cover local, remote, and ambiguous remote cases.