fix: wt new resumes existing worktrees instead of erroring

[ukanga]

Closes #19

What changed

wt new <name> now behaves idempotently when a worktree already exists:

• Registered worktree — prints Worktree '<name>' already exists at <path>, entering it. to stderr, then drops into the shell (or prints the path for --print-path). The migration step is skipped, so no stash is created and the main repo's branch is not switched.
• Orphan directory (exists on disk but not tracked by git) — exits non-zero with an actionable error naming the path and suggesting git worktree prune or manual removal. The directory is not modified.
• Fresh create — unchanged behaviour.

Commits

• fix(worktree): replace bare path.exists() bail in create_worktree with a two-branch check against git worktree list
• fix(new): intercept in cmd_new before the migration step; reuse existing get_worktree_info + spawn_wt_shell
• docs: update README and commands/do.md

Tests

Three new integration tests in tests/new_resume_test.rs covering fresh, already-registered, and orphan states. The orphan test confirms the directory is left untouched after the error.