Refactor daemon execution and preflight flow

[shinpr]

Summary

• Introduce typed runner command errors and use them for supervisor retry classification
• Move setup and acceptance skeleton preflight into dedicated packages with owned domain types and focused tests
• Add run artifact helpers, executor flow helpers, task status helpers, and shared runner option helpers
• Clean up PR command parsing, artifact names, schema/reference checks, VCS evidence helpers, and plugin metadata

Verification

• go test ./...
• git diff --check
• rg checks for removed legacy aliases/artifacts and runartifact domain type leakage