Support external v1 harness plugins

[sethkarten]

Summary

Adds generic v1 support for external harness packages without vendoring their implementation into public Verifiers.

• allows flat external plugin modules to be selected by normalized id
• preserves internal plugin import errors instead of masking missing dependencies as missing plugins
• adds a Prime runtime wait fallback that polls get() if wait_for_creation() times out and continues when the sandbox is actually RUNNING
• documents the generic external plugin contract in architecture docs
• adds regression tests for external harness loading, custom config forwarding, exact label preservation, and the Prime wait fallback

This lets research teams install private harness packages alongside Verifiers while keeping harness source in private repositories.

Testing

uv run --isolated --no-project   --with-editable .   --with pytest   --with pytest-asyncio   --with renderers==0.1.8.dev43   python -m pytest tests/v1/test_loaders.py

uv run --isolated --no-project --with ruff ruff check   verifiers/v1/ARCHITECTURE.md   verifiers/v1/loaders.py   verifiers/v1/runtimes/prime.py   tests/v1/fixtures/external_harness_v1.py   tests/v1/test_loaders.py

git diff --check