Feat/telemetry http exporter (#314)

* feat(telemetry): add OTLP HTTP exporter and print TraceID

- Add HTTP/protobuf exporter support alongside existing gRPC exporter
- Route based on OTEL_EXPORTER_OTLP_PROTOCOL config (http/protobuf vs grpc)
- Print TraceID to stderr when telemetry is enabled for easier correlation
- Add corresponding unit tests

* feat(telemetry): add span coverage for LLM calls, tool execution, plan/filter phases

- Add StartLLMSpan / RecordLLMResult helpers (span.go), symmetric with
  existing StartToolSpan / RecordToolResult
- Wrap LLM completion calls in llmloop.RunPerFile with llm.request spans
- Wrap all three tool execution paths in executeToolCall with
  tool.execute.* spans (dynamic tools, code_comment sync/async, other tools)
- Add plan.execute span around executePlanPhase
- Add main.loop span around RunPerFile call in executeSubtask
- Add review_filter.execute span around executeReviewFilter, with
  comments.before / comments.filtered attributes
- Record llm.error attribute on LLM failures for diagnosability
- Record review.repo / review.from / review.to / review.model on the
  top-level review.run span
- Metrics (RecordLLMRequest / RecordToolCall) are preserved alongside
  the new spans — they serve different purposes (aggregate dashboards
  vs per-run diagnosis)

Verified end-to-end against Sunfire (OTLP HTTP gateway): full span tree
observed for review.run -> subtask.execute -> plan.execute/main.loop/
review_filter.execute -> llm.request/tool.execute.*

* fix(telemetry): address CR findings — span error handling, async span lifecycle, protocol robustness

- Add span.RecordError(err) to RecordLLMResult and RecordToolResult for
  consistency with EndSpan
- Use OTel standard pattern (span.SetStatus + span.RecordError) in error
  paths of review.run, plan.execute, main.loop, review_filter.execute
- Move async code_comment span end into pool.Submit callback so span
  duration reflects actual execution time
- Unify time.Since(startTime) in code_comment error path to a single dur
- Remove http/json from supported OTLP protocols (not actually implemented)
- Add stderr warning when unknown OTLP protocol falls back to gRPC

* feat(telemetry): include trace_id in JSON output, restrict stderr to text format

- Add trace_id as top-level field in jsonOutput struct (omitempty)
- JSON format: trace_id in structured response for programmatic extraction
- Text format: TraceID printed to stderr for human debugging
- Telemetry disabled: trace_id field omitted entirely

* fix: address PR review findings

- loop.go: wrap async span lifecycle in defer to prevent leak on panic
- exporter.go: update parseOTLPEndpoint comment to reflect gRPC+HTTP usage
- scan_cmd.go: align traceID extraction and OTel error handling with review_cmd
- output.go/shared.go: propagate traceID to outputJSONNoFiles for consistency
- agent.go: move comments.filtered attribute before early return so 0 is
  distinguishable from not-executed

* feat(telemetry): address PR review — http/json routing, LLM span coverage, trace_id tests

- Route http/json to HTTP exporter (Go OTel SDK HTTP transport only
  supports protobuf serialization; users need HTTP transport, not JSON encoding)
- Add llm.request spans to executePlanPhase, executeReviewFilter, and
  ReLocateComment with Usage nil-safety consistent with loop.go
- Add trace_id assertions to output helper tests and emitRunResult
  end-to-end tests using real TracerProvider

* docs: add OTLP protocol selection and endpoint format to telemetry section

Sync across all 5 README language versions (en, zh-CN, ja-JP, ko-KR, ru-RU).

* fix: unify time.Since in async code_comment defer to single dur variable

cmd/opencodereview/shared.go

@@ -276,8 +276,10 @@ func emitRunResult(
 		telemetry.RecordCommentsGenerated(ctx, int64(len(comments)))
 	}
 
+	traceID := telemetry.TraceIDFromContext(ctx)
+
 	if outputFormat == "json" && len(comments) == 0 && ag.FilesReviewed() == 0 {
-		return outputJSONNoFiles()
+		return outputJSONNoFiles(traceID)
 	}
 
 	// Agent-text audiences need stdout back before PrintTraceSummary so the
@@ -296,7 +298,7 @@ func emitRunResult(
 		return outputJSONWithWarnings(comments, ag.Warnings(), ag.FilesReviewed(),
 			ag.TotalInputTokens(), ag.TotalOutputTokens(), ag.TotalTokensUsed(),
 			ag.TotalCacheReadTokens(), ag.TotalCacheWriteTokens(), duration,
-			ag.ProjectSummary(), ag.ToolCalls())
+			ag.ProjectSummary(), ag.ToolCalls(), traceID)
 	}
 	outputTextWithWarnings(comments, ag.Warnings())
 	if summary := ag.ProjectSummary(); summary != "" {