Add a Codex SessionEnd memory adapter

[janitor-manager]

Parent: #7

Add the Codex-native adapter after a stable, safely filterable conversation export contract is available.

Current blocker: official Codex hook documentation exposes SessionEnd but states that transcript_path format is not a stable hook interface.

Acceptance:

• Bind only to the documented Codex SessionEnd contract.
• Parse a stable documented/exported conversation schema, not rollout internals.
• Require explicit enablement and agent_id.
• Filter non-conversation/tool/credential material, then call the retry-safe session ingest API.
• Add deterministic fixtures and installation/disable/failure documentation.