wt new/session add fail when target is the default branch already at the primary worktree

[ukanga]

Repro

From the repo root on main:

$ wt session add main
Creating worktree: main
Error: Failed to create worktree: Preparing worktree (checking out 'main')
fatal: 'main' is already used by worktree at '/home/me/code/wt'

$ wt new main
Error: Failed to create worktree: Preparing worktree (checking out 'main')
fatal: 'main' is already used by worktree at '/home/me/code/wt'

Why it fails

Both commands shell out to git worktree add <path> main. Git refuses because main is already checked out at the repo root (the primary worktree). The error surfaces git's internal message rather than something wt-shaped.

Expected

The default branch is, by definition, already a "worktree" — it's the primary one. The commands should recognize this and reuse the existing primary worktree path instead of trying to create a duplicate:

• wt session add main → add a tmux window/session pointing at the repo root.
• wt new main → either drop into a shell at the repo root, or print a friendly hint pointing the user at the primary worktree.

Suggested fix

WorktreeManager::list_worktrees() already exposes the primary worktree as the entry with empty task_id (src/worktree_manager.rs:357-364). A small helper — "is this branch checked out at the primary worktree?" — lets both call sites short-circuit:

• ensure_worktree_path (src/session_cmd.rs:132-155): if the requested name matches the primary worktree's branch, return repo.root instead of calling create_worktree.
• cmd_new (src/main.rs:156-211): same check; reuse repo.root instead of creating.

Generalizes naturally to "any branch already checked out at the primary worktree," not just main/master.

Open question

wt session add main is a clear win — useful, and the current failure is git-shaped rather than wt-shaped.

wt new main is less obvious. wt new semantically creates a worktree, and elsewhere in the codebase the primary worktree is explicitly not a wt worktree (pick_worktree filters out empty task_id at src/main.rs:295-298; cmd_use rejects "main" at src/main.rs:412-414). One option is to replace the git error here with a friendlier wt-level error pointing the user at wt session add main or cd <repo-root>, rather than pretending main is a wt worktree. Open to either direction.