fix(server): suffix-fallback auto-named branches and rotate opencode waterfall

Auto-namer now probes refs/heads/<name> and appends -2, -3, … (up to 50)
when the desired slug is taken, instead of failing with "branch already
exists" and leaving the worktree on its placeholder name.

Structured-generation waterfall replaces opencode/gpt-5-nano with
opencode/minimax-m2.5-free and opencode/nemotron-3-super-free so users
without nano configured fall through to free models they actually have.

packages/server/src/utils/checkout-git.ts

@@ -870,6 +870,10 @@ async function getWorktreePathForBranch(cwd: string, branchName: string): Promis
   }
 }
 
+export async function localBranchExists(cwd: string, branchName: string): Promise<boolean> {
+  return doesGitRefExist(cwd, `refs/heads/${branchName}`);
+}
+
 export async function renameCurrentBranch(
   cwd: string,
   newName: string,