Fetch origin before stck push

[brdv]

Summary

Ensure stck push refreshes remote refs before planning and execution.

This reduces stale-ref risk and aligns push behavior with status/sync freshness expectations.

Changelist

• src/cli.rs
  • run_push now runs git fetch origin before loading/reusing push state.
  • Push exits early with actionable error if fetch fails.
• tests/cli_skeleton.rs
  • Added regression test push_shows_fetch_failure_remediation.

Validation

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