test: expand CI core suite with config/context contracts

[danbot315]

P0 follow-up to expand CI checks while keeping the suite green.

What changed

• Expanded CI test step to include additional core contract tests:
  • packages/core/src/workflow-generator.test.ts
  • packages/core/src/context.test.ts
  • packages/core/src/config.test.ts

Stabilization fixes included

• Restored context/config compatibility behavior expected by test suite:
  • DotGithubContext root/output path defaults and legacy compatibility
  • setConfigPath/getConfigPath override behavior (path or directory)
  • addActionToConfig supports legacy rootDir string call pattern and normalizes absolute output paths
• Fixed plugin generator strict/type issues introduced during earlier refactors.
• Made workflow construct registration tolerant for test doubles lacking addWorkflow.

Validation

• bun run build
• bun run typecheck
• bun run lint
• bun run test -- packages/core/src/workflow-generator.test.ts packages/core/src/context.test.ts packages/core/src/config.test.ts