fix: support episode decay evidence snapshots

[cafitac]

Summary

• Fix decay-collapse evidence snapshots for episode candidates by reading source_ids_json instead of the fact/procedure-only evidence_ids_json column.
• Add a regression test covering episode decay-collapse preview without raw content leakage or mutation.
• Record the next-step dogfood results: metadata-rich dogfood-only epoch passed, default ranking remains protected, reviewed-candidate promotion remains narrow-only, and broad G4/background apply remains blocked.

Validation

• uv run --python 3.11 pytest tests/test_cli.py::test_dogfood_decay_collapse_preview_handles_episode_source_ids_without_mutation tests/test_cli.py::test_dogfood_decay_collapse_preview_reports_stale_weak_evidence_without_mutation tests/test_cli.py::test_dogfood_g5h_next_brainlike_steps_are_read_only_or_guarded tests/test_cli.py::test_dogfood_background_dry_run_broad_g4_apply_contract_red_until_supported -q
• uv run --python 3.11 pytest tests/ -q
• uv run --python 3.11 pytest tests/test_roadmap_contract.py tests/test_cli.py::test_dogfood_decay_collapse_preview_handles_episode_source_ids_without_mutation tests/test_cli.py::test_dogfood_decay_collapse_preview_reports_stale_weak_evidence_without_mutation tests/test_cli.py::test_dogfood_g5h_next_brainlike_steps_are_read_only_or_guarded -q