Add SessionDB state helpers and migration contracts

[offendingcommit]

What changed

• add lazy, ownership-aware SessionDB opening and injection
• add typed helpers for reading/listing sessions, reading messages, and appending structured messages
• validate migration and persistence against a generated legacy state.db using the real hermes-agent checkout
• document connection ownership and safe temporary-database testing
• report the tested upstream hermes-agent commit in CI

Why

Plugin consumers need a small convention-correct state access seam without duplicating SQLite schemas or taking ownership of Hermes-managed connections. Contract coverage ensures the helpers continue to follow the real SessionDB migration and operation behavior as upstream evolves.

Validation

• 5 SessionDB helper unit tests pass
• generated legacy state.db migration contract passes against hermes-agent 3d7dda4cf42176d587b459345c56236a26030324
• package wheel builds successfully
• git diff --check passes
• hosted unittest and hermes-contract jobs will provide Linux validation