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/lib/utils.ts

@@ -36,3 +36,8 @@ export function formatTokens(n: number): string {
   if (n >= 1_000) return `${(n / 1_000).toFixed(1)}k`;
   return String(n);
 }
+
+/** Build an issue URL using the human-readable identifier when available. */
+export function issueUrl(issue: { id: string; identifier?: string | null }): string {
+  return `/issues/${issue.identifier ?? issue.id}`;
+}