Spec 34: Chunk priority

[noahbfreedman-cloud]

Summary

• add a chunk priority manager with static and learned priority signals
• use priority-aware selection when a token budget is provided
• add CLI visibility and tests for budgeted chunk selection

Testing

• python3 -m pytest tests/test_priority.py tests/test_hooks.py tests/test_chunk_defaults.py tests/test_keywords_cli.py -q