Rename the change routes from /requests to /changes

Why

The metadata tree and the interface now say changes, but the browser still uses /r/<repo>/requests/... for the list, the conversation page, the forms, and every redirect and activity link. With no public release yet, there is no reason to keep the old spelling: rename the routes and drop the URL mismatch instead of carrying an alias.

Scope

• /r/<repo>/requests... becomes /r/<repo>/changes... for every method: list, new, view, edit, files, runs, live, comment form, issue search, approvals, merges, comments, state, target, and issue links.
• Every URL the server builds follows: redirects, activity and search URLs, commit and code-tree links, the number alias redirect, CLI page links, and templates.
• The repository header’s tab key becomes changes so the active tab still matches.
• No compatibility redirect from /requests: the route is gone.
• The internal names (Request type, request_id, handler names, template files) stay; this is a route rename, not an entity rename.

Checklist

• Update server/routes.go and the number-alias redirect.
• Update every URL builder, redirect, and template link.
• Update the CLI’s page links (who.Page("changes", ...)).
• Update the route tests and the terminology note in docs/README.md.
• Leave no /requests link in templates or generated activity/search URLs.

Acceptance criteria

• The change list, page, edit form, files, runs, live updates, comments, approvals, merge, state, target, and linked-issue routes all answer under /r/<repo>/changes..., and the old /requests... paths return 404.
• The Changes tab is active on every change page.
• Activity, search, the number alias, and CLI output link to /changes....
• vole workflow check and the verify workflow pass.