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>

docs/PLAN.md

@@ -23,11 +23,14 @@ Sentry-like frontend error collector.
 
 ## Phase 2 — Recent issues for the web UI
 
-- [ ] Fingerprint spans with `status=ERROR`/exception events and logs with
+- [x] Fingerprint spans with `status=ERROR`/exception events and logs with
       `severity >= ERROR` into deduped Issues (count, firstSeen, lastSeen,
       sample entries, affected services), updated incrementally on ingest.
-- [ ] `GET /api/issues`, `GET /api/issues/{id}`.
-- [ ] UI "Issues" tab (lives in the separate frontend/`dist` project).
+      `internal/issues` (fingerprinting/dedup) + `internal/collector/issue_extract.go`
+      (OTLP field extraction).
+- [x] `GET /api/issues`, `GET /api/issues/{id}`.
+- [ ] UI "Issues" tab (lives in the separate frontend/`dist` project — this
+      repo's API is ready for it).
 
 ## Phase 3 — Frontend error collector (Sentry-style)