`SandboxManager.ensure_workspace` calls `fetch_base_ref` (then
`worktree add origin/<ref>`) and `fetch_pr_head` (then
`worktree add --detach FETCH_HEAD`). Both used to issue a plain
`git fetch origin <ref>`. On a `--filter=blob:none` pool the fetch
inherits `remote.origin.partialclonefilter` from the pool config and
brings the commit + tree but no blobs. The next `worktree add` runs
in a non-token subprocess, hits a missing blob, and tries a lazy
promisor fetch — which under `ProxyGitTransport` deployments has no
PAT in the orchestrator container and dies with
fatal: could not read Username for 'https://github.com'
fatal: could not fetch <sha> from promisor remote
`git_ops.fetch_ref` and `fetch_pr_head` now pass `--refetch
--no-filter` so the fetch (which already runs through the token-bearing
transport) eagerly materializes every blob reachable from the requested
ref. `--refetch` is required: without it git short-circuits on "we
already have this commit" and the lazy-fetch path stays primed.
`fetch_prune` (the periodic pool refresh) is untouched, so the
partial-clone disk savings are preserved on the steady-state path.
`remote.origin.partialclonefilter` is left intact in the pool config.
Fixes#1818
roboomp
a692ffeb31