Closes #1777

Fixes the upload staging failure from build-with-wordpress run 29307978522. Provider diagnostics and stacks that mention OpenAiProvider or WP_Agents_Registry now remain reviewable, while PHP source is detected using PHP structure even when disguised as .json or .txt.

Control request, result, and native-input artifacts continue through existing path sanitization, secret redaction, and source-tree/path exclusion. The normalized result stages before declared reviewer artifacts, so a declared-artifact rejection preserves the failure result.

How to test

1. Run npm run build.
2. Run npm run test:agent-task-contracts.
3. Run npm run test:redaction.
4. Run npm run test:production-boundary-enforcement && npm run test:runtime-tool-policy.
5. Run WP_CODEBOX_RUN_NETWORK_INTEGRATION=1 npm run test:runtime-sources-playground-integration.
6. Run actionlint .github/workflows/run-agent-task.yml.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode with openai/gpt-5.6-terra
• Used for: Collaboratively inspected the failure, implemented the structural detector and regression fixtures, and ran verification.