fix(ci): check out API-created dep branch via remote-tracking ref

git fetch origin <branch> only sets FETCH_HEAD, so the silent checkout
failure left commits on the wrong ref and the push failed. Fetch into
refs/remotes/origin/<branch> and checkout -B from it.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

docs/specs/DEVX-170.md

@@ -0,0 +1,33 @@
+# DEVX-170: create_dependency_pr checks out the API-created branch
+
+## Problem
+
+After DEVX-169 the dependency branch is created via `POST /branches`, but
+`git fetch origin <branch>` only populates `FETCH_HEAD`. The follow-up
+`git checkout <branch>` fails silently (`check=False`), commits land on
+the wrong ref, and `git push origin <branch>` fails with
+`src refspec does not match any`. Observed creating the sso_bridge 0.4.1
+infra dependency PR.
+
+## Approach
+
+REQ-1: Fetch the API-created branch into `refs/remotes/origin/<branch>`
+and force-create the local branch with `git checkout -B <branch>
+origin/<branch>`, both with `check=True` so failures surface.
+
+## Test Plan
+
+- Unit test asserts the fetch refspec and `checkout -B` invocations.
+
+## Deploy Plan
+
+devx release tag; producers pick it up via pin bumps.
+
+## Rollback Plan
+
+Revert; dep-PR creation stays broken (status quo).
+
+## Acceptance Criteria
+
+- [x] REQ-1: the clone checks out the API-created branch so commit and
+  push target `deps/<pkg>-<version>`; covered by unit tests.