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/Inbox.tsx

@@ -299,7 +299,7 @@ export function Inbox() {
                           <button
                             type="button"
                             className="truncate text-muted-foreground transition-colors hover:text-foreground"
-                            onClick={() => navigate(`/issues/${issue.id}`)}
+                            onClick={() => navigate(`/issues/${issue.identifier ?? issue.id}`)}
                           >
                             {issue.identifier ?? issue.id.slice(0, 8)} · {issue.title}
                           </button>
@@ -372,7 +372,7 @@ export function Inbox() {
                 <div
                   key={issue.id}
                   className="flex cursor-pointer items-center gap-3 px-4 py-3 transition-colors hover:bg-accent/50"
-                  onClick={() => navigate(`/issues/${issue.id}`)}
+                  onClick={() => navigate(`/issues/${issue.identifier ?? issue.id}`)}
                 >
                   <Clock className="h-4 w-4 shrink-0 text-muted-foreground" />
                   <PriorityIcon priority={issue.priority} />