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/ActiveAgentsPanel.tsx

@@ -359,7 +359,7 @@ function AgentRunCard({
         <div className="px-3 py-1.5 border-b border-border/40 text-xs flex items-center gap-1 min-w-0">
           <span className="text-muted-foreground mr-1">Working on:</span>
           <Link
-            to={`/issues/${run.issueId}`}
+            to={`/issues/${issue?.identifier ?? run.issueId}`}
             className="text-blue-400 hover:text-blue-300 hover:underline min-w-0 truncate"
             title={issue?.title ? `${issue?.identifier ?? run.issueId.slice(0, 8)} - ${issue.title}` : issue?.identifier ?? run.issueId.slice(0, 8)}
           >

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)}

ui/src/components/IssueProperties.tsx

@@ -211,7 +211,7 @@ export function IssueProperties({ issue, onUpdate }: IssuePropertiesProps) {
         {issue.parentId && (
           <PropertyRow label="Parent">
             <Link
-              to={`/issues/${issue.parentId}`}
+              to={`/issues/${issue.ancestors?.[0]?.identifier ?? issue.parentId}`}
               className="text-sm hover:underline"
             >
               {issue.ancestors?.[0]?.title ?? issue.parentId.slice(0, 8)}

ui/src/components/NewIssueDialog.tsx

@@ -130,7 +130,7 @@ export function NewIssueDialog() {
         title: `${issue.identifier ?? "Issue"} created`,
         body: issue.title,
         tone: "success",
-        action: { label: `View ${issue.identifier ?? "issue"}`, href: `/issues/${issue.id}` },
+        action: { label: `View ${issue.identifier ?? "issue"}`, href: `/issues/${issue.identifier ?? issue.id}` },
       });
     },
   });