Task 1: Repo scaffold + graph models

[rahlk]

Part of epic #1 — Task 1 of the v1 plan (.superpowers/plans/2026-07-07-cocoa-v1-system-graph.md, local scratch).

Goal: pyproject (uv, pinned deps), .gitignore, LICENSE (MIT), README stub, and cocoa/system/models.py: Provenance (DERIVED-STATIC/INFERRED), NodeKind, EdgeKind, Node, Edge, Skipped, SystemGraph with save/load round-trip.
Done when: test/test_models.py passes.