docs: add model/README.md — Python perimeter policy and setup

model/README.md

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+# model
+
+Python + PyTorch perimeter. This is the only directory in the project that uses Python.
+
+## Contents
+
+```
+model/
+  train.py              TS-JEPA training entry point
+  configs/              YAML configs per phase
+    phase0-mae.yaml
+    phase1-tsjepa.yaml
+  tsjepa/               TS-JEPA model implementation (adapted from paper)
+  tests/                pytest tests for model components
+  requirements.txt      pinned Python dependencies
+  .venv/                (gitignored — created by `task model:setup`)
+```
+
+## Setup
+
+```bash
+task model:setup    # creates .venv and installs requirements via uv
+```
+
+## Dependency policy
+
+Every Python dependency must be justified in a comment in `requirements.txt`. Prefer Go implementations for anything outside the training loop. When adding a new dependency, add an entry to DECISIONS.md explaining why a Go alternative wasn't sufficient.
+
+## Python version
+
+3.12 (pinned in `.python-version`)