chore: add cross-repo checklist to PR template and teach ship-pr to use it

[fullstackjam]

What does this PR do?

Adds a cross-repo checklist section to the PR template and updates the ship-pr skill to read the template before opening a PR instead of using a hardcoded body.

Why?

When a change spans multiple repos (e.g. CLI feature + docs update in openboot.dev), it's easy to forget the other repo. A checklist in the PR template surfaces the question at the right moment — for both humans and agents.

The ship-pr skill was previously writing its own --body unconditionally, which would silently discard the template. It now reads .github/pull_request_template.md first and fills in its sections.

Testing

• go vet ./... passes
• Relevant tests added or updated
• Tested locally (./openboot install --dry-run or similar)

Cross-repo checklist

• Does this need a docs/content update in openboot.dev?
• Does this change the CLI ↔ server API contract? (update openboot-contract first)

Notes for reviewer

Skill files under .claude/skills/ are not Go code — no unit tests apply. The change is verified by reading the before/after of the two modified files.