Derive deduplicated error issues from traces and logs (Phase 2 backend)

- Add internal/issues.Tracker: fingerprints (kind, service, title) into
  deduplicated Issues with count/firstSeen/lastSeen/affected services and
  bounded sample occurrences, evicting the least-recently-seen issue once
  a cap is reached.
- Add internal/collector/issue_extract.go to pull issues out of ingested
  OTLP data: spans with an error status (preferring exception.type/message
  from an "exception" event, falling back to the span status message), and
  logs at ERROR/FATAL severity (by severity number or text).
- Wire extraction into Collector.ConsumeTraces/ConsumeLogs and expose
  GET /api/issues and GET /api/issues/{id}.
- Fix a pre-existing data race in the collector test's mockStore (unsynced
  access to entries while addToHistory appends asynchronously), found via
  `go test -race` while adding the new issue-tracking tests.
- Update docs/PLAN.md, README.md and agents_example.md for the new API.

The UI "Issues" tab itself lives in the separate frontend project that
builds this server's dist/ bundle; this commit only covers the backend.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

cmd/server/main.go

@@ -206,6 +206,8 @@ func main() {
 	uiMux.HandleFunc("/api/history/search", coll.HandleSearch)
 	uiMux.HandleFunc("/api/history/wait", coll.HandleWait)
 	uiMux.HandleFunc("GET /api/traces/{traceId}", coll.HandleTraceByID)
+	uiMux.HandleFunc("/api/issues", coll.HandleIssues)
+	uiMux.HandleFunc("GET /api/issues/{id}", coll.HandleIssueByID)
 	uiMux.HandleFunc("/ws", wsServer.HandleConnections)
 	uiMux.Handle("/", spaHandler(http.Dir("./dist")))