Add `sc create --closes` support for issue-closing PR metadata

Goal

Allow a branch/PR to declare issues it closes, e.g.

sc create --closes 1212

Then sc submit should include the appropriate GitHub closing keyword in the PR body so merging the PR closes the referenced issue.

Open design questions

• Should --closes support multiple issue numbers?
• Should the data live in a Shortcake-* trailer, another durable branch marker, or only in generated PR body content?
• How should this interact with existing stack description regeneration on sc submit?
• Should sc create --closes owner/repo#123 be supported for cross-repo issues?

Acceptance criteria

• sc create --closes <issue> persists the closing reference durably enough for later sc submit.
• sc submit adds closing keywords to created/updated PR descriptions without losing the stack map.
• Existing PR body sync remains idempotent.
• Tests cover single and multiple issue references.

Start at the sc create and sc submit command entry points, then trace the existing stack description and PR body synchronization behavior. Review the existing tests for PR body sync and add coverage for durable single and multiple issue references, preserving the stack map and idempotence.