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>

server/src/routes/activity.ts

@@ -47,6 +47,21 @@ export function activityRoutes(db: Db) {
     res.status(201).json(event);
   });
 
+  // Resolve issue identifiers (e.g. "PAP-39") to UUIDs
+  router.param("id", async (req, res, next, rawId) => {
+    try {
+      if (/^[A-Z]+-\d+$/i.test(rawId)) {
+        const issue = await issueSvc.getByIdentifier(rawId);
+        if (issue) {
+          req.params.id = issue.id;
+        }
+      }
+      next();
+    } catch (err) {
+      next(err);
+    }
+  });
+
   router.get("/issues/:id/activity", async (req, res) => {
     const id = req.params.id as string;
     const issue = await issueSvc.getById(id);