Sprint 5F: artifact chunk compile integration

[samrusani]

Summary

• add optional artifact retrieval to POST /v0/context/compile
• return artifact chunks in a separate context-pack section with deterministic ordering and summary metadata
• trace included, limit-excluded, and non-ingested artifact decisions

Verification

• ./.venv/bin/python -m pytest tests/unit
• ./.venv/bin/python -m pytest tests/integration
• ./.venv/bin/python -m pytest tests/integration/test_context_compile.py
• git diff --check