Summary

• Advertises FeatureRunSession for the modal provider now that acquire, reuse, keep, and cleanup paths are wired.
• Returns a RunSessionHandle after modal sandbox identity exists, including kept leases, sync-only paths, and keep-on-failure retained leases.
• Adds focused coverage for capability advertisement, kept session output, keep-on-failure session output, and the generated provider matrix entry.

Verification

• go test ./internal/providers/modal ./internal/providers/all
• scripts/check-docs.sh
• git diff --check

Refs #182