fix: show relative worktree paths in interactive picker

[ahmedelgabri]

Summary

• Fix format_worktree_for_display() to show relative paths (e.g. feature/my-thing) instead of just the basename (my-thing) in the interactive fzf picker
• Pass bare_root from build_worktree_display_list() so workspace name is computed as a relative path from bare root
• Add 4 tests covering interactive round-trip and display format verification for slash-containing worktrees

Before/After

For worktree at <bare-root>/feature/my-thing:

	Display
Before	my-thing [feature/my-thing] ~/repo/feature/my-thing
After	feature/my-thing [feature/my-thing] ~/repo/feature/my-thing

Simple worktrees like main are unchanged.

Test plan

• shellcheck git-wt passes
• All 104 bats tests pass (4 new)
• nix fmt -- no formatting changes
• Pre-commit and pre-push hooks pass

Closes #11