fix: repair worktree starts from origin/<default-branch>

When repairing a stale polecat worktree, start from origin/<default-branch>
instead of the bare repo's HEAD. This ensures repaired polecats have the
latest fetched commits rather than potentially stale code.

- Add WorktreeAddFromRef to git package for creating worktrees from a ref
- RepairWorktreeWithOptions now uses origin/<default-branch> as start point
- Respects rig's configured default branch (main, master, etc.)

Based on PR #112 by markov-kernel. Test changes from that PR can be
submitted separately.

Closes #101

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: markov-kernel <markov-kernel@users.noreply.github.com>

internal/git/git.go

@@ -523,6 +523,13 @@ func (g *Git) WorktreeAdd(path, branch string) error {
 	return err
 }
 
+// WorktreeAddFromRef creates a new worktree at the given path with a new branch
+// starting from the specified ref (e.g., "origin/main").
+func (g *Git) WorktreeAddFromRef(path, branch, startPoint string) error {
+	_, err := g.run("worktree", "add", "-b", branch, path, startPoint)
+	return err
+}
+
 // WorktreeAddDetached creates a new worktree at the given path with a detached HEAD.
 func (g *Git) WorktreeAddDetached(path, ref string) error {
 	_, err := g.run("worktree", "add", "--detach", path, ref)