fix(v0.1): scope policies by action and honor empty conditions

[t3chn]

Summary

• Add optional action scoping to PolicySpec and the policy JSON schema.
• Treat an empty when list as unconditional (all([]) == true) instead of never-fired.
• Add verifier regression tests for scoped policies, cross-action non-firing, warn policies, and unconditional action-scoped blocks.

Issue

• Related: lab exp-004 / q-001 authoring-cost boundary 2 findings.

Test plan

• ruff check src tests
• mypy src
• pytest
• python3 scripts/check_docs.py
• PYTHONPATH=src python3 examples/v0_1/demo.py

Notes

• This PR is intentionally based on the rebased main after PR feat(v0.1): add governed gateway with compensation trace #1, so the diff contains only the action-scoping and empty-when semantic fixes.