Fix evergreen agent instructions to create local tracking branch before push

The push_to_pull_request_branch tool requires a named local branch to create
patches from. When the agent checks out the PR branch with a detached HEAD,
the tool fails with "Branch does not exist locally". Updated step 2 of the
agent instructions to explicitly use `git checkout -b` to create a local
tracking branch.

Agent-Logs-Url: https://github.com/githubnext/tsessebe/sessions/d2700d0e-7bbb-4650-aa27-84a4b45b7c29

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot

.github/workflows/evergreen.md

@@ -279,7 +279,11 @@ A pre-flight step has already identified a PR that needs attention. Read the sel
    - `selected.base_branch` — the target branch (usually `main`)
    - `selected.attempts` — how many times we've already tried on this SHA
 
-2. **Check out the PR branch** and investigate the issues.
+2. **Check out the PR branch** as a local tracking branch so the push tool can find it:
+   ```bash
+   git checkout -b <head_branch> origin/<head_branch>
+   ```
+   where `<head_branch>` is `selected.head_branch` from the selection file. **Do not** use a detached HEAD checkout — the `push-to-pull-request-branch` tool requires a named local branch.
 
 3. **Fix the issues**: