feat: add G5e decay collapse preview

[cafitac]

Summary

• Add dogfood decay-collapse-preview as the G5e read-only bridge from decay-risk scoring to human review.
• Emit ref-safe decay_collapse_candidates, quality gate, automation policy, and explicit no-mutation collapse review guardrails.
• Document the new dogfood surface in docs/hermes-dogfood.md.

Test Plan

• .venv/bin/python -m pytest tests/test_cli.py::test_dogfood_decay_collapse_preview_reports_stale_weak_evidence_without_mutation -q (RED first: command missing)
• .venv/bin/python -m pytest tests/test_cli.py::test_dogfood_trace_cluster_preview_reports_ref_safe_clusters_without_mutation tests/test_cli.py::test_dogfood_reinforcement_refinement_preview_scores_repeated_activation_without_mutation tests/test_cli.py::test_dogfood_decay_collapse_preview_reports_stale_weak_evidence_without_mutation tests/test_cli.py::test_python_module_cli_activations_decay_risk_reports_ref_safe_resolution_hints -q
• .venv/bin/python -m pytest tests/test_repository_trust_docs.py -q
• .venv/bin/python -m pytest tests/ -q
• Local live read-only smoke against /Users/reddit/.agent-memory/memory.db: /tmp/agent-memory-g5e-live-smoke.json, verified read_only=true, mutated=false, default retrieval unchanged, no protected table count changes, and no raw prompt/query-preview markers.

Safety

• Preview only: no review queue writes, no deprecate/delete/collapse mutation, no ordinary-conversation auto-approval, no retrieval ranking changes.
• Any future mutation remains a separate guarded apply policy with backup/audit/approval/rollback.