[codex] Add workload selection compatibility examples

[pawelchcki]

What

Add dd-compile-policy examples documenting workload selection compatibility for future tracer injection and Kubernetes selector payloads.

The examples cover:

• future enum names for INJECT_TRACERS and K8s selector evaluators
• opaque target metadata carried through existing string fields
• notes on target-shaped policy fields that old compilers reject

Why

The Agent passes APM workload selection RC payloads through dd-compile-policy. These examples make the compatibility behavior explicit: unknown object fields are rejected, while unknown enum names compile to their *_UNKNOWN values and become no-ops on old runtimes.

Validation

• git diff --check
• Compiled future_action_and_k8s_selectors.json with dd-compile-policy v0.1.2 in Docker
• Compiled opaque_target_metadata.json with dd-compile-policy v0.1.2 in Docker

[datadog-prod-us1-5]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 21.66%

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