Add stable runtime invocation helpers

[chubes4]

Summary

• Add public wp_agent_dispatch_ability() and wp_agent_run_runtime_package() helpers so generated/runtime host code can use the bootstrap API instead of internal file paths or classes.
• Document the stable runtime bundle import and invocation boundary.
• Extend the runtime package smoke test to cover the public host helper.

Tests

• php tests/runtime-package-run-contract-smoke.php
• php tests/runtime-agent-bundle-importer-smoke.php
• php tests/no-product-imports-smoke.php
• composer smoke
• composer phpstan

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Investigating existing runtime package/import APIs, implementing the public helper boundary, updating docs/tests, and running verification.