feat(tracing): add OpenTelemetry tracing to entrypoint step execution

[waveywaves]

Feature request

Add OTel tracing to the entrypoint binary so each step's execution lifecycle is visible in traces. Currently the entrypoint has zero OTel support — no TracerProvider, no spans, no trace context propagation.

Use case

The gap between controller-side traces and actual step execution is the biggest blind spot in Tekton observability. Operators can see that a TaskRun was created but not how long each step waited, ran, or wrote results.

Implementation hints

pkg/entrypoint/entrypointer.go:

• Go() (line 198) — top-level orchestrator, root entrypoint span
• readResultsFromDisk() (line 437) — result extraction + SPIRE signing
• applyStepResultSubstitutions() — runtime variable substitution

cmd/entrypoint/:

• waiter.go:52 — Wait() polls for predecessor step completion
• runner.go:73 — Run() executes user command via exec.CommandContext
• post_writer.go:33 — Write() writes completion signal files

Prerequisites (large effort):

1. Inject TRACEPARENT/TRACESTATE env vars into step containers during pod construction (pkg/pod/pod.go)
2. Initialize OTel TracerProvider in entrypoint main() with OTLP exporter
3. Create child spans linking to the TaskRun reconciler's parent span

Tracked in umbrella #9701 as "Large effort."

Part of umbrella: #9701