Match recall scopes by complete path components

[importcpp]

Summary

• compare recall scopes against complete path components
• prevent a scope such as experience from matching a sibling domain such as experience-archive
• add a regression test for the colliding-prefix case

Closes #17

Testing

• uv run pytest -q --cov=agent_memory --cov-report=term-missing --cov-fail-under=85 (389 passed, 90.91% coverage)
• uv run ruff check .
• uv run mypy