Cache or retry WordPress release metadata startup asset resolution

## Problem
Several Lab WP Codebox runs failed before the agent task could execute because Playground startup could not fetch WordPress release metadata:

`Unable to resolve Playground startup asset wordpress-release-metadata for WordPress latest from https://api.wordpress.org/core/version-check/1.7/?channel=beta: fetch failed`

This showed up during WPSG Lab pilots while switching provider paths. It is independent of the model/provider choice and turns transient network/metadata fetch failures into full task failures.

## Expected
WP Codebox should make this startup path more resilient for Lab cooking:

- cache WordPress release metadata/startup assets between runs when safe,
- retry transient fetch failures with useful backoff,
- optionally allow the runner/plan to pin a known WordPress version or use a local cached release asset,
- surface a concise diagnostic that distinguishes infrastructure fetch failure from agent/model failure.

## Evidence From Today
Observed in WPSG pilot runs including:

- `site-generation-loop-20260615c`
- `site-generation-loop-20260615e`

Representative diagnostic:

`agent_runtime.workload: Unable to resolve Playground startup asset wordpress-release-metadata for WordPress latest from https://api.wordpress.org/core/version-check/1.7/?channel=beta: fetch failed`

Artifacts referenced by Homeboy included WP Codebox artifact directories such as:

- `/home/chubes/Developer/.tmp/homeboy-wp-codebox-artifacts-C7Pk4Q`
- `/home/chubes/Developer/.tmp/homeboy-wp-codebox-artifacts-tzyP4h`

## Why This Matters
For Lab cooking, startup metadata fetch should not be a frequent blocker. It also obscures whether the actual provider/model path is working because the failure happens before useful agent execution evidence exists.

## Acceptance Criteria
- Transient `api.wordpress.org` metadata fetch failures are retried or served from a safe cache.
- The final diagnostic includes whether the failure was cache miss, network fetch failure, or invalid metadata.
- A plan/runtime option exists to pin or reuse a cached WordPress release for repeatable Lab runs.

Start at the Playground startup asset resolution path and the agent_runtime.workload diagnostic shown in the issue; reproduce the metadata fetch failure using the referenced WPSG pilot runs if available. Define the safe cache, retry, pinning, and diagnostic behavior from the acceptance criteria, then verify that transient failures are distinguishable from cache misses, network failures, invalid metadata, and agent/model failures.

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