Add a Prime post-session memory adapter

[janitor-manager]

Parent: #7

Add a Prime-native adapter when a documented lifecycle event and stable conversation artifact contract are available.

Acceptance:

• Bind to a documented post-session lifecycle event.
• Read only its exact supplied stable conversation artifact.
• 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.