Add generic workspace preload artifact contract

[chubes4]

Summary

• Register the generic agent-runtime/workspace-preload package artifact type in Agents API.
• Add a core-safe validation/import contract for repository preload payloads without cloning, mounting, persistence, or product-specific policy.
• Cover the contract with a pure-PHP smoke test.

Owner decision

Agents API is the correct first upstream owner for the portable artifact contract because it already owns generic package artifact registration and adoption callbacks. Runtime materialization remains outside Agents API: WP Codebox should consume this contract when preparing sandbox workspaces, and Homeboy Extensions/WPSG should only pass package metadata.

Verification

• php tests/workspace-preload-artifact-smoke.php
• php tests/package-capability-contract-smoke.php
• php tests/package-adoption-orchestration-smoke.php
• php -l src/Packages/class-wp-agent-workspace-preload-artifact.php
• php -l tests/workspace-preload-artifact-smoke.php
• php -l agents-api.php

Downstream adoption

• WP Codebox: consume agent-runtime/workspace-preload package artifacts during runtime bundle import/session preparation and materialize declared repositories into sandbox workspace mounts.
• WPSG: move extension payloads from extensions/datamachine-code/workspace_preload/*.json to a generic extensions/agent-runtime/workspace-preload/*.json path and drop the compatibility_adapter metadata once WP Codebox materialization lands.
• Homeboy Extensions: continue treating this as runtime/provider-side package metadata; no Data Machine or WPSG-specific logic belongs in generic orchestration.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Inspected upstream boundaries, drafted the generic artifact contract and smoke tests, and prepared the PR. Chris remains responsible for review and merge.