figure out remote branch properly with v6

[Soph]

Turns out the old code worked with v5 but was not really correct FETCH_HEAD is not a direct ref, this should make it work with go-git v6 again

[cursor]

PR Summary

Low Risk
Small, localized change to git ref lookup during fetch+merge; main risk is incorrect ref resolution for some remotes/branches causing sync failures.

Overview
Fixes session-log branch sync to read the fetched remote tip from the remote-tracking ref (refs/remotes/<remote>/<branch>) instead of FETCH_HEAD, and uses that hash as the second parent when creating the merge commit.

This updates the error messaging accordingly and should restore correct behavior with go-git/v6 where FETCH_HEAD is not a normal reference.

^{Written by Cursor Bugbot for commit dbc5e43. Configure here.}

[Copilot]

Pull request overview

Adjusts the session-branch fetch+merge recovery logic to work correctly with go-git v6 by avoiding reliance on FETCH_HEAD (which isn’t a normal ref that go-git can resolve).

Changes:

• Resolve the remote commit via the remote-tracking ref (refs/remotes/<remote>/<branch>) instead of FETCH_HEAD.
• Use the remote-tracking ref hash as the second parent when creating the merge commit.