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)

internal/polecat/manager.go

@@ -304,6 +304,9 @@ func (m *Manager) Recreate(name string, force bool) (*Polecat, error) {
 	// Prune stale worktree entries
 	_ = repoGit.WorktreePrune()
 
+	// Fetch latest from origin to ensure we have fresh commits
+	_ = repoGit.Fetch("origin")
+
 	// Delete the old branch so worktree starts fresh from current HEAD
 	// Ignore error - branch may not exist (first recreate) or may fail to delete
 	_ = repoGit.DeleteBranch(branchName, true)
@@ -316,10 +319,13 @@ func (m *Manager) Recreate(name string, force bool) (*Polecat, error) {
 
 	// Create worktree - handle both cases like Add() does
 	if branchExists {
-		// Branch still exists, create worktree using existing branch
-		// This happens if delete failed (e.g., protected branch)
+		// Branch still exists after deletion attempt - force-reset to origin/main
+		// This ensures the polecat starts with fresh code, not stale commits
+		if err := repoGit.ResetBranch(branchName, "origin/main"); err != nil {
+			return nil, fmt.Errorf("resetting stale branch to origin/main: %w", err)
+		}
 		if err := repoGit.WorktreeAddExisting(polecatPath, branchName); err != nil {
-			return nil, fmt.Errorf("creating worktree with existing branch: %w", err)
+			return nil, fmt.Errorf("creating worktree with reset branch: %w", err)
 		}
 	} else {
 		// Branch was deleted, create fresh worktree with new branch from HEAD