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.

flake.lock

@@ -0,0 +1,27 @@
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+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "00455b0a3690d3f5dc61e9aef4277dc86235b73f",
+        "type": "github"
+      },
+      "original": {
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+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
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+        "nixpkgs": "nixpkgs"
+      }
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+  "root": "root",
+  "version": 7
+}