Provide clear error when remote is missing in `publish`, `track`, `delete --remote`

[alexrinass]

The publish, track, and delete --remote commands require a remote to function but don't validate upfront that one exists. Instead they let the underlying git call fail with a confusing low-level error (e.g. "does not appear to be a git repository").

Locations

• cmd/publish.go:158, 173 — git.Fetch() warns on failure but git.PushBranch() hard-fails with a cryptic error
• cmd/track.go:104 — git.Fetch() hard-fails with a raw git error
• cmd/delete.go:145 — git.DeleteRemoteBranch() fails when remote doesn't exist

Expected Behavior

These commands should check git.RemoteExists(remote) early and return a clear error message like:

Error: No remote 'origin' configured. Cannot publish branch.

Related: #69, PR #74