feat(tracing): add semantic span attributes to reconciler child spans

Feature request

22 of 26 child spans in the PipelineRun/TaskRun reconcilers carry zero attributes — they are unqueryable timers. Add semantic attributes (outcome, error reason, task/pipeline ref, step count) to key spans.

Use case

Operators cannot filter or search traces by outcome ("show me all failed createPod spans") because spans lack queryable attributes. Adding semantic attributes enables Jaeger/Tempo filtering and alerting on specific span patterns.

Implementation hints

Spans missing attributes (all have zero today):

In pkg/reconciler/pipelinerun/pipelinerun.go:

• reconcile, resolvePipelineState, runNextSchedulableTask, createTaskRuns — add pipeline name, task count, status

In pkg/reconciler/taskrun/taskrun.go:

• reconcile, createPod, stopSidecars, failTaskRun — add task name, step count, failure reason

Also fix:

• Deduplicate identical SetAttributes calls between initTracing root spans and ReconcileKind spans (attributes set twice per reconcile)
• Use OTel semantic conventions (k8s.namespace.name) instead of ad-hoc keys ("namespace") to align with semconv v1.40.0 bump in #9697

Pattern:

span.SetAttributes(
    attribute.String("tekton.pipeline.name", pipelineName),
    attribute.Int("tekton.task.count", len(tasks)),
    attribute.String("tekton.status", string(condition.Reason)),
)

Part of umbrella: #9701

good first issue — no new spans needed, just add attributes to existing ones.