test(core): assert memory summary prompt injection

[cbusillo]

Summary

• adds a focused core test that seeds $CODE_HOME/memories/memory_summary.md
• asserts the initial model request includes the memory section, sentinel memory text, and dedup marker path

Validation

• cargo test -p code-core --test prompt_context_dedup initial_session_request_injects_memory_when_summary_exists -- --nocapture
• ./build-fast.sh