feat: explain fact review decisions

[cafitac]

Summary

• add agent-memory review explain fact to combine fact status, default retrieval visibility, same claim-slot alternatives, transition history, and replacement chain into one forensic decision context
• reuse existing transition history and supersedes/replaces relation data without changing default approved-only retrieval behavior
• document the new review surface in README, install-smoke docs, and current handoff

Test Plan

• uv run pytest tests/test_cli.py::test_python_module_cli_review_explain_fact_shows_decision_context -q
• uv run pytest tests/test_review_and_scope_ranking.py tests/test_cli.py -q
• uv run pytest tests/ -q
• uv run python scripts/check_release_metadata.py
• uv run python scripts/smoke_release_readiness.py
• npm pack --dry-run
• git diff --check
• node --check bin/agent-memory.js
• static diff scan found 0 credential findings