Support host-resolved phased plugin inputs

[chubes4]

Summary

• add an explicit pluginInput recipe contract that consumes one declared typed JSON artifact
• project JSON Pointer fields into bounded generic plugin descriptors
• resolve immutable archives or latest stable WordPress.org packages on the host
• mount, activate, and verify projected plugins in the same live runtime before later workflow steps continue
• preserve stage-specific phase evidence, interruption handling, and cleanup
• add recipe-run --output with bounded JSON file output and a long-run activity heartbeat
• make the source-checkout launcher choose build:release only when the checkout provides it

Fixes #2414.

Consumer: Automattic/static-site-importer#1427.

Verification

• npm run build
• npx tsx tests/recipe-phased-plugin-input.test.ts
• npx tsx tests/recipe-runtime-setup-staged-materialization.test.ts
• npx tsx tests/recipe-run-summary-output.test.ts
• npx tsx tests/recipe-run-artifacts-mount-guard.test.ts
• npx tsx scripts/source-checkout-entrypoint-smoke.ts
• focused SSI runner and one-runtime contract tests: 5 passed
• completed SSI 1260-cascade-proof: one runtime, one succeeded fixture, zero findings, pixel-identical browser output, and 8/8 valid editor blocks
• repeated one-runtime batch execution ranged from 161.6s to 459.0s under host contention, versus 589.6s for the prior discovery plus import runtimes

Contract Notes

• ambient stdout and key-name matching cannot supply package inputs
• package resolution remains host-side; the WordPress guest does not download providers
• empty projected manifests continue without package activation
• malformed artifacts and resolution, mount, activation, or readiness failures retain their phased stage

AI Assistance

OpenAI GPT-5.6 Sol through OpenCode traced the recipe lifecycle, implemented the typed phased-input and output-file contracts, added focused tests, ran the cross-repository browser proof, and prepared this pull request. Chris Huber directed the architecture and reviewed the evidence.