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>

agents_example.md

@@ -16,6 +16,8 @@ This document provides instructions for AI agents on how to utilize the OTEL Col
         - **Ideal for Automation**: Use this after triggering an action to wait for the exact result.
         - **Rate limited**: capped per client IP; back off if you receive `429`.
     - `GET /api/traces/{traceId}`: Fetch every stored entry (spans and correlated logs) referencing a specific trace id in one call. Prefer this over substring-searching for a trace id.
+    - `GET /api/issues?limit={limit}`: List deduplicated error issues (from error-status spans and error/fatal-severity logs), most recently seen first. Use this to triage what's currently broken before diving into raw search.
+    - `GET /api/issues/{id}`: Fetch a single issue's detail, including sample occurrences with trace ids you can feed into `/api/traces/{traceId}`.
     - `GET /api/history`: Fetch recent history of all telemetry (searches the full persistent store, not just recent in-memory entries).
     - `GET /api/history/traces`: Fetch recent traces.
     - `GET /api/history/logs`: Fetch recent logs.