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

[patrick91]

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.