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/components/CommandPalette.tsx

@@ -149,7 +149,7 @@ export function CommandPalette() {
             <CommandSeparator />
             <CommandGroup heading="Issues">
               {issues.slice(0, 10).map((issue) => (
-                <CommandItem key={issue.id} onSelect={() => go(`/issues/${issue.id}`)}>
+                <CommandItem key={issue.id} onSelect={() => go(`/issues/${issue.identifier ?? issue.id}`)}>
                   <CircleDot className="mr-2 h-4 w-4" />
                   <span className="text-muted-foreground mr-2 font-mono text-xs">
                     {issue.identifier ?? issue.id.slice(0, 8)}