feat: add observation review candidates

[cafitac]

Summary

• add read-only observations review-candidates for top retrieval observation refs
• include fact review explanations, replacement-chain signals, graph summaries, and follow-up commands
• document the dogfood review workflow and refresh the current handoff

Test Plan

• TMPDIR=$PWD/.tmp-test uv run pytest tests/test_cli.py::test_python_module_cli_observations_review_candidates_explains_top_refs_without_mutation_or_raw_queries -q
• TMPDIR=$PWD/.tmp-test uv run pytest tests/test_cli.py::test_python_module_cli_observations_audit_reports_frequent_and_stale_refs_without_raw_queries tests/test_cli.py::test_python_module_cli_observations_review_candidates_explains_top_refs_without_mutation_or_raw_queries tests/test_cli.py::test_python_module_cli_review_explain_fact_shows_decision_context tests/test_cli.py::test_python_module_cli_graph_inspect_returns_read_only_relation_neighborhood -q
• PYTHONPATH=src uv run python -m agent_memory.api.cli observations review-candidates --help
• TMPDIR=$PWD/.tmp-test 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
• temp-DB observations review-candidates smoke with secret-like query strings; verified no secret-like text in output
• static diff secret scan: finding_count: 0