fix(stack): update existing branch pointer when reusing branch name

CRITICAL FIX: When pushing to a stack with --branch specifying an existing
branch name, we were:
1. Skipping branch creation (correct)
2. Adding metadata entry pointing to new commit (correct)
3. BUT leaving the Git branch pointing to old commit (BUG!)

This caused stack corruption on next ca sync/ca validate, which would
'fix' the metadata back to the stale commit, dropping user's work.

Now we call update_branch_to_commit() to fast-forward the existing branch
to the new commit, keeping Git refs and metadata in sync.

Scenario:
- User: ca push --branch feat-auth <commit1>
- User: ca push --branch feat-auth <commit2>  # Reusing same name
- Before: feat-auth branch still at commit1, metadata at commit2 → corruption
- After: feat-auth branch updated to commit2, metadata at commit2 → consistent

Author: JAManfrediDK

src/stack/manager.rs

@@ -468,7 +468,19 @@ impl StackManager {
         // 🆕 CREATE ACTUAL GIT BRANCH from the specific commit
         // Check if branch already exists
         if self.repo.branch_exists(&branch) {
-            // Branch already exists, skip creation
+            // Branch already exists - update it to point to the new commit
+            // This is critical: if we skip this, the branch points to the old commit
+            // but metadata points to the new commit, causing stack corruption
+            self.repo
+                .update_branch_to_commit(&branch, &commit_hash)
+                .map_err(|e| {
+                    CascadeError::branch(format!(
+                        "Failed to update existing branch '{}' to commit {}: {}",
+                        branch,
+                        &commit_hash[..8],
+                        e
+                    ))
+                })?;
         } else {
             // Create the branch from the specific commit hash
             self.repo