Fallback to origin for missing sync old-base refs

[brdv]

Summary

Make sync resilient when a planned old-base branch no longer exists locally by falling back to origin/<branch>.

Changelist

• src/gitops.rs
  • Added resolve_old_base_for_rebase(base_branch):
    • tries refs/heads/<base> first
    • falls back to refs/remotes/origin/<base>
    • returns actionable error if neither resolves
• src/cli.rs
  • run_sync now uses resolve_old_base_for_rebase instead of local-head-only resolution.
• tests/cli_skeleton.rs
  • Extended git stub to simulate missing local refs and custom remote SHAs.
  • Added regression test sync_uses_remote_old_base_when_local_old_base_is_missing.

Validation

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features