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/pages/ApprovalDetail.tsx

@@ -149,7 +149,7 @@ export function ApprovalDetail() {
             (linkedIssues?.length ?? 0) > 1
               ? "Review linked issues"
               : "Review linked issue",
-          to: `/issues/${primaryLinkedIssue.id}`,
+          to: `/issues/${primaryLinkedIssue.identifier ?? primaryLinkedIssue.id}`,
         }
       : linkedAgentId
         ? {
@@ -236,7 +236,7 @@ export function ApprovalDetail() {
               {linkedIssues.map((issue) => (
                 <Link
                   key={issue.id}
-                  to={`/issues/${issue.id}`}
+                  to={`/issues/${issue.identifier ?? issue.id}`}
                   className="block text-xs rounded border border-border/70 px-2 py-1.5 hover:bg-accent/20"
                 >
                   <span className="font-mono text-muted-foreground mr-2">