agent task examples need a half-configured provider-stack regression guard

## Problem

WP Codebox examples and runtime contract correctly show that provider stacks need explicit top-level fields on the agent-task input: `provider`, `model`, `provider_plugin_paths`, and `secret_env`. In practice, orchestrators repeatedly regress into a half-configured state where they pass `provider_plugin_paths` but omit `provider` and/or `secret_env`.

That state is confusing because the provider plugin gets mounted, but provider selection/credential exposure is not guaranteed by WP Codebox. The failure then appears later inside the embedded runtime as a provider/OAuth failure rather than as a clear contract problem at recipe/request construction time.

Today's WPSG typed-artifact Lab loop hit this exact shape: provider plugin path present, but no top-level `provider`, `model`, or Codex `secret_env`. The runtime then reached Data Machine/Codex and failed with `Codex OAuth refresh failed.` Regenerating the plan with the full WP Codebox contract made Homeboy fail fast correctly on missing runner secret refs before runtime execution.

## Existing good contract

The cookbook example already shows the right shape in `examples/recipes/cookbook/codex-agent-smoke.json`:

- provider plugin mounted in `inputs.extra_plugins`
- `inputs.secretEnv` includes the five `AI_PROVIDER_OPENAI_CODEX_*` names
- workflow args include `provider=codex` and `model=gpt-5.5`

The runtime-core builder also has the right top-level fields: `provider`, `model`, `provider_plugin_paths`, and `secret_env`.

## Expected

Add an explicit regression guard/diagnostic around half-configured provider stacks so orchestrators get a clear signal before this reaches an embedded runtime.

Possible acceptable fixes:

- Add a smoke test that builds an agent-task recipe from input with `provider_plugin_paths` but no `provider`/`secret_env` and asserts the output/diagnostic makes the missing contract obvious.
- Add a non-secret diagnostic/warning in recipe/task metadata when `provider_plugin_paths` is non-empty but provider or secret env names are absent.
- Strengthen docs/tests around the cookbook example so provider plugin paths are never treated as provider selection or credential exposure.

This should stay generic: WP Codebox should not hardcode Codex-specific behavior outside the cookbook example. The generic lesson is that mounting a provider plugin is not the same as selecting a provider or exposing credentials.

## Acceptance criteria

- Automated coverage prevents a provider-plugin-only agent-task input from looking fully configured.
- The diagnostic or test names the missing generic fields: `provider` and `secret_env`.
- No secret values are accepted or logged.
- Do not run Cargo locally for this fix; WP Codebox verification should use its normal Node/TypeScript tests or CI/Lab as appropriate.

## Related context

This is a recurring integration regression between orchestrators and WP Codebox. The Homeboy/Homeboy Extensions side is tracked separately; this issue is for WP Codebox examples/contract guardrails so future callers do not repeat the same half-configured provider stack.

Start with examples/recipes/cookbook/codex-agent-smoke.json and the runtime-core agent-task builder described in the issue. Run the normal Node/TypeScript tests, then add coverage for input containing provider_plugin_paths without provider or secret_env. Done means the diagnostic or assertion names both missing generic fields, does not expose secret values, and keeps provider behavior generic.

Written by the indexing model from the issue text.