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/components/ActiveAgentsPanel.tsx

@@ -359,7 +359,7 @@ function AgentRunCard({
         <div className="px-3 py-1.5 border-b border-border/40 text-xs flex items-center gap-1 min-w-0">
           <span className="text-muted-foreground mr-1">Working on:</span>
           <Link
-            to={`/issues/${run.issueId}`}
+            to={`/issues/${issue?.identifier ?? run.issueId}`}
             className="text-blue-400 hover:text-blue-300 hover:underline min-w-0 truncate"
             title={issue?.title ? `${issue?.identifier ?? run.issueId.slice(0, 8)} - ${issue.title}` : issue?.identifier ?? run.issueId.slice(0, 8)}
           >