Cold launch fetches exactly the requested ref; delete the workspace-clone fetch

[blooop]

Question

Per the #144 decision, the foreground cold path makes exactly one narrow network call and no broad one. Depends on the updater-sweep build ticket (the sweep must exist before the foreground stops sweeping).

• ensure_repo (repo_manager.py:204-236) stops fetching: remove the _should_fetch/fetch_repo branch and the auto_fetch parameter (production never passes False; tests that did get the behavior for free). It becomes the pure locked clone-if-missing primitive.
• ensure_branch (workspace_clone.py:172-206): replace lazy_fetch (:190) with an unconditional targeted fetch of the requested ref into the bare repo (git fetch origin +refs/heads/<branch>:refs/heads/<branch>); on ref-missing-on-remote, one targeted fetch of the default branch before ensure_branch_exists bases on it. Model the outcome as the three-way result from Move the interval git fetch off the foreground launch path and out of the repo lock #144 (updated / ref-missing / network-failed → proceed-fresh / create-from-default / warn-and-use-cache-or-error), not a bool. Do not write last_fetched here.
• Delete _prepare_workspace step 4 (workspace_clone.py:298-310) — its output is unused (plain git checkout <branch> at :338).
• Do not restructure lock cycles — Collapse the duplicated ensure_repo passes and repo-lock cycles on the cold path #143 owns that seam.

Failing tests first: (1) the lock-aware fetch pin from #144 — no +refs/heads/* fetch in the foreground, only the targeted refspec ever under the repo lock (flock LOCK_NB probe in the fake subprocess.run); (2) re-registration path runs zero fetch in the workspace clone; (3) staleness contract integration: remote gains a commit after the cache cloned → new workspace HEAD == remote tip; ref only on remote → succeeds; ref nowhere → existing error.

Files: devlaunch/worktree/repo_manager.py, devlaunch/worktree/workspace_clone.py, test/test_worktree_repo_manager.py (existing auto_fetch tests), test/integration/test_repo_manager_real.py.