Rewrite as a probabilistic model with walk-forward evaluation.

Replace the 2024 model (model.py, ~2000 lines) with the btcmodel package, the
baseline for future work:

- Forecasts are quantiles of log price at each horizon, scored with CRPS in a
  walk-forward backtest (origins every 30 days from 2014, horizons 1 month to
  4 years). Skill is relative to a zero-drift random walk, with circular
  block-bootstrap intervals and a count of independent windows.
- Development data stops at 2024-11-26, the last day the 2024 model saw.
  Later outcomes are a holdout, scored only by `backtest --holdout`.
- Models: random_walk, drift_rw, and cycle (the 2024 model's cycle-position
  drift, now kernel-smoothed and recency-weighted). On development data
  nothing beats the random walk with confidence; cycle loses at every horizon.
- Prices: the Investing.com archive moves to data/ (cut at 2024-11-26; its
  last row was intraday) and is extended with Coinbase daily closes by
  `update`.

Also: Nix flake dev shell (Python 3.13, pandas 3), ruff in place of black,
pytest suite, and a rewritten README. NOTES.md is removed as inaccurate, and
poetry is dropped.

pyproject.toml

@@ -1,18 +1,24 @@
-[tool.poetry]
-name = "bitcoin-model"
-version = "0.1.0"
-description = ""
-authors = ["Anthropic Claude <[email protected]>", "Sam Fredrickson <[email protected]>"]
+[project]
+name = "btcmodel"
+version = "0.2.0"
+description = "Probabilistic Bitcoin price model"
+authors = [
+    { name = "Anthropic Claude", email = "[email protected]" },
+    { name = "Sam Fredrickson", email = "[email protected]" },
+]
 readme = "README.md"
+requires-python = ">=3.13"
+dependencies = ["numpy>=2", "pandas>=3", "scipy>=1.14", "matplotlib>=3.9"]
 
-[tool.poetry.dependencies]
-python = "^3.13"
-pandas = "^2.2.3"
-matplotlib = "^3.9.2"
-seaborn = "^0.13.2"
-scipy = "^1.14.1"
+[dependency-groups]
+dev = ["pytest", "ruff"]
 
+[tool.ruff]
+line-length = 100
 
-[build-system]
-requires = ["poetry-core"]
-build-backend = "poetry.core.masonry.api"
+[tool.ruff.lint]
+select = ["E", "F", "I", "B", "UP"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["."]