fix: make /ship reruns resume safely

[AliFozooni]

Summary

• add a rerun-safety checkpoint before the /ship version bump so a failed PR-creation step resumes instead of double-bumping VERSION
• teach the push and PR/MR steps to detect already-pushed branches and existing PRs/MRs before recreating them
• add regression coverage for the generated ship skill text so the resume-safe behavior stays locked in

Problem

A partial failure between push and PR creation can currently leave /ship in a state where rerunning the workflow bumps VERSION again and duplicates the top CHANGELOG entry.

Testing

• git diff --check
• node verification of the new resume-safety strings in ship/SKILL.md, ship/SKILL.md.tmpl, and test/gen-skill-docs.test.ts

Fixes #649