Support issue identifiers (PAP-39) in URLs and prefer them throughout

Backend:
- Add router.param middleware in issues, activity, and agents routes to
  resolve identifiers (e.g. PAP-39) to UUIDs before handlers run
- Simplify GET /issues/:id now that param middleware handles resolution
- Include identifier in getAncestors response and issuesForRun query
- Add identifier field to IssueAncestor shared type

Frontend:
- Update all issue navigation links across 15+ files to use
  issue.identifier ?? issue.id instead of bare UUIDs
- Add URL redirect in IssueDetail: navigating via UUID automatically
  replaces the URL with the human-readable identifier
- Fix childIssues filter to use issue.id (UUID) instead of URL param
  so it works correctly with identifier-based URLs
- Add issueUrl() utility in lib/utils.ts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/context/LiveUpdatesProvider.tsx

@@ -100,7 +100,7 @@ function resolveIssueToastContext(
     ref,
     title,
     label: title ? `${ref} - ${truncate(title, 72)}` : ref,
-    href: `/issues/${issueId}`,
+    href: `/issues/${cachedIssue?.identifier ?? issueId}`,
   };
 }