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/Dashboard.tsx

@@ -266,7 +266,7 @@ export function Dashboard() {
                     <div
                       key={issue.id}
                       className="px-4 py-2 text-sm cursor-pointer hover:bg-accent/50 transition-colors"
-                      onClick={() => navigate(`/issues/${issue.id}`)}
+                      onClick={() => navigate(`/issues/${issue.identifier ?? issue.id}`)}
                     >
                       <div className="flex gap-3">
                         <div className="flex items-start gap-2 min-w-0 flex-1">