Add dirty worktree check before merge to prevent unsaved changes from being lost

src/cli.ts

@@ -413,6 +413,12 @@ program
   .description("Merge a branch into main and close the session")
   .action(async (branch: string) => {
     const root = await git.repoRoot()
+    const session = await state.getSession(root, branch)
+
+    if (session && await git.isDirty(session.worktree)) {
+      console.error(`✖ Branch "${branch}" has unsaved changes. Run "sandlot save ${branch}" first.`)
+      process.exit(1)
+    }
 
     const conflicts = await git.merge(branch, root)