fix: add backup before git reset --hard in rebase_continue and rebase_skip

rebase_continue() and rebase_skip() performed git reset --hard during
squash-merge Reset handling without creating a backup branch first,
unlike rebase() which always creates a backup. This was a data loss risk.

Add create_backup_branch() helper and call it before destructive resets
in both functions. Also add 10 new integration tests covering squash-merge
handling in continue/skip, second conflicts, abort with multiple modified
branches, step mode with conflict, dirty working directory, cleanup
backups after continue, and status with mixed statuses.

Author: dashed

src/git_chain/operations.rs

@@ -601,6 +601,11 @@ impl GitChain {
                             parent_name.bold()
                         );
 
+                        // Create backup before destructive reset
+                        self.create_backup_branch(&state.chain_name, &branch_name)?;
+                        let backup_name = format!("backup-{}/{}", state.chain_name, &branch_name);
+                        println!("📦 Created backup branch: {}", backup_name.bold());
+
                         let command = format!("git reset --hard {}", parent_name);
                         let output = Command::new("git")
                             .arg("reset")
@@ -910,6 +915,11 @@ impl GitChain {
                             parent_name.bold()
                         );
 
+                        // Create backup before destructive reset
+                        self.create_backup_branch(&state.chain_name, &branch_name)?;
+                        let backup_name = format!("backup-{}/{}", state.chain_name, &branch_name);
+                        println!("📦 Created backup branch: {}", backup_name.bold());
+
                         let command = format!("git reset --hard {}", parent_name);
                         let output = Command::new("git")
                             .arg("reset")
@@ -1122,6 +1132,15 @@ impl GitChain {
         }
     }
 
+    /// Create a backup branch for a named branch in a chain.
+    fn create_backup_branch(&self, chain_name: &str, branch_name: &str) -> Result<(), Error> {
+        let (object, _reference) = self.repo.revparse_ext(branch_name)?;
+        let commit = self.repo.find_commit(object.id())?;
+        let backup_branch = format!("backup-{}/{}", chain_name, branch_name);
+        self.repo.branch(&backup_branch, &commit, true)?;
+        Ok(())
+    }
+
     /// Delete backup branches for a chain after successful rebase.
     fn cleanup_backup_branches(&self, chain_name: &str, branches: &[BranchState]) {
         let mut cleaned = 0;