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/Issues.tsx

@@ -157,7 +157,7 @@ export function Issues() {
               key={issue.id}
               identifier={issue.identifier ?? issue.id.slice(0, 8)}
               title={issue.title}
-              onClick={() => navigate(`/issues/${issue.id}`)}
+              onClick={() => navigate(`/issues/${issue.identifier ?? issue.id}`)}
               leading={
                 // eslint-disable-next-line jsx-a11y/click-events-have-key-events, jsx-a11y/no-static-element-interactions
                 <div className="flex items-center gap-2" onClick={(e) => e.stopPropagation()}>
@@ -220,7 +220,7 @@ export function Issues() {
                   key={issue.id}
                   identifier={issue.identifier ?? issue.id.slice(0, 8)}
                   title={issue.title}
-                  onClick={() => navigate(`/issues/${issue.id}`)}
+                  onClick={() => navigate(`/issues/${issue.identifier ?? issue.id}`)}
                   leading={
                     // eslint-disable-next-line jsx-a11y/click-events-have-key-events, jsx-a11y/no-static-element-interactions
                     <div className="flex items-center gap-2" onClick={(e) => e.stopPropagation()}>