Improve agent-facing history endpoints (Phase 1)

- Fix /api/history/{type} to search the full persistent store instead of
  only the bounded in-memory ring buffer, so filtered history isn't
  silently truncated.
- Add structured type/since/until filters to /api/history/search and
  /api/history/wait, alongside the existing free-text query.
- Add GET /api/traces/{traceId} to fetch all entries for a trace in one
  call instead of relying on substring search.
- Rate-limit /api/history/wait per client IP.
- Add docs/PLAN.md tracking the phased improvement plan; update README
  and agents_example.md for the new/changed endpoints.

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

cmd/server/main.go

@@ -205,6 +205,7 @@ func main() {
 	uiMux.HandleFunc("/api/history/logs", coll.HandleHistoryByType("logs"))
 	uiMux.HandleFunc("/api/history/search", coll.HandleSearch)
 	uiMux.HandleFunc("/api/history/wait", coll.HandleWait)
+	uiMux.HandleFunc("GET /api/traces/{traceId}", coll.HandleTraceByID)
 	uiMux.HandleFunc("/ws", wsServer.HandleConnections)
 	uiMux.Handle("/", spaHandler(http.Dir("./dist")))