feat: add restore apply contract checkpoint

[cafitac]

Summary

• Add a read-only blocked restore apply contract checkpoint for dogfood query-preview-cleanup-restore --apply.
• Require the separate restore policy, actor, and reason while exposing only reason_sha256 in output.
• Keep live restore unavailable with restore_apply_contract_checkpoint_only and live_restore_not_implemented blocked reasons.
• Update G4a handoff/roadmap docs to record the contract-only restore apply boundary.

Tests

• uv run pytest tests/test_cli.py::test_python_module_cli_dogfood_query_preview_cleanup_restore_dry_run_validates_artifact_without_mutation_or_leaks tests/test_cli.py::test_python_module_cli_dogfood_query_preview_cleanup_restore_apply_is_contract_blocked_without_mutation_or_leaks tests/test_cli.py::test_python_module_cli_dogfood_query_preview_cleanup_restore_dry_run_blocks_source_database_mismatch tests/test_cli.py::test_python_module_cli_dogfood_query_preview_cleanup_restore_dry_run_reports_wrong_policy_as_read_only_error tests/test_cli.py::test_python_module_cli_dogfood_query_preview_cleanup_restore_dry_run_blocks_artifact_integrity_mismatch -q
• uv run pytest tests/ -q
• uv run ruff check src tests
• git diff --check
• local smoke: /tmp/agent-memory-restore-apply-contract-smoke

Safety

• Restore apply is not implemented and cannot mutate the DB.
• Broad G4 apply remains blocked.
• Raw query previews and raw reason text are not printed or stored.
• Future live restore still requires a separate disposable-restore rehearsal/audit implementation slice.