Implement prompt context memory rollout

[pcharbon70]

Summary

• adds the jido_memory-backed ContextMemory adapter with scope policy, config defaults, lifecycle cleanup, and provider validation
• wires bounded prompt-memory retrieval and capture into the conversation runtime
• documents the prompt-memory/provenance/durable-memory boundary and adds focused integration coverage

Verification

• mix compile
• mix test test/jido_code/conversations/context_memory_test.exs test/jido_code/phase_seventy_eight_integration_test.exs
• mix docs (completed with existing type-reference warnings in lib/jido_code/error.ex)