Show an issue's linked changes and commits in the sidebar

What

The issue detail page now keeps its references in the side column:

• Referenced in changes links each change request whose closes list names the issue, showing its number, state, author, and age.
• Referenced in commits moves the commit backlink table out of the main column and into the same compact side-list shape the overview uses.

Why

The main column is for the issue description and discussion. References are context, so they belong beside it with labels and assignees. Listing the linked changes also answers “is someone already fixing this?” without leaving the issue.

Implementation

• forge/index: add RequestsByID, sharing the request-row scanner with Requests.
• server/web: reverse-look up closes links through the index and expose them on the issue page view.
• server/web/templates: move commits into the aside and add the linked-change rows.

Testing

• ./vole workflow check
• ./vole workflow run verify — format, vet, build, licenses, govulncheck, packaging, markdown links, and the test suite all passed.