fix(spawn): reset stale polecat branches to origin/main

When recreating a polecat worktree, the branch deletion might fail
silently, leaving a stale branch with old commits. The worktree would
then be created from this stale branch, missing recent main changes.

Fix:
- Add ResetBranch() method to git package for force-updating branches
- Fetch from origin before recreation to ensure fresh commits
- If branch deletion fails, force-reset the branch to origin/main
- This ensures polecats always start with the latest code

Discovered during gt-8tmz swarm when nux polecat was missing the
internal/formula/ directory that was added after its branch diverged.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/git/git.go

@@ -401,6 +401,13 @@ func (g *Git) DeleteBranch(name string, force bool) error {
 	return err
 }
 
+// ResetBranch force-updates a branch to point to a ref.
+// This is useful for resetting stale polecat branches to main.
+func (g *Git) ResetBranch(name, ref string) error {
+	_, err := g.run("branch", "-f", name, ref)
+	return err
+}
+
 // Rev returns the commit hash for the given ref.
 func (g *Git) Rev(ref string) (string, error) {
 	return g.run("rev-parse", ref)