fix: migrate retrieval observations lazily

[cafitac]

Summary

• fix observation log lazy migration for existing pre-v0.1.35 databases
• ensure observations list and observation writes create retrieval_observations if an older DB lacks the table
• add regression coverage for listing observations against a legacy DB shape

Test Plan

• uv run pytest tests/test_cli.py::test_python_module_cli_observations_list_migrates_existing_database_without_observation_table tests/test_cli.py::test_python_module_cli_retrieve_observe_records_secret_safe_local_observation -q -> 2 passed
• real legacy DB smoke: drop retrieval_observations, run agent-memory observations list, parse JSON -> ok
• uv run pytest tests/ -q -> 168 passed
• uv run python scripts/check_release_metadata.py -> ok
• uv run python scripts/smoke_release_readiness.py -> ok
• npm pack --dry-run -> ok
• git diff --check -> ok
• node --check bin/agent-memory.js -> ok
• static diff secret scan -> finding_count: 0