Provide exact completion report shape

[chubes4]

Summary

• generate a complete lane-specific v1 completion-report example from the caller source delta
• require compact valid JSON with explicit serialization rules
• preserve caller IDs and source refs while directing the model to replace evidence and diff-dependent values

Root cause

Both hosted acceptance runs reached completion validation but emitted syntactically invalid JSON because the model-facing prompt listed fields without a concrete nested object shape.

Reproductions:

• https://github.com/Automattic/agents-api/actions/runs/29741779974
• https://github.com/Automattic/build-with-wordpress/actions/runs/29741769561

How to test

1. Run php tests/validate-docs-agent-packages.php.
2. Run php tests/validate-docs-agent-completion-contract.php.
3. Run php tests/validate-external-native-package-sources.php.
4. Check out WP Codebox a6fe2d208e990a8d04104aa74aacbb8d1539fbc1 and run WP_CODEBOX_DIR=/path/to/wp-codebox php tests/validate-wp-codebox-run-agent-task-contract.php.
5. Run actionlint .github/workflows/*.yml.

Compatibility

This strengthens only the generated model prompt. The v1 completion schema, validator semantics, workflow inputs, package bytes, and consumer contracts are unchanged.

Closes #172

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode with GPT-5.6-sol
• Used for: Diagnosed hosted report failures and added the deterministic JSON shape and assertions with Chris reviewing and owning the change.