Defuse the clone-and-run trap: tracked pipeline data warns loudly

Eric spotted it: the README told people to clone this repo, rm the
committed data, and run — which writes THEIR pipeline artifacts at
git-TRACKED paths. The next `git pull` (this repo commits data every
session) refuses to merge, and the internet's standard remedies for
that error — reset --hard, checkout ., stash, clean -fdx — destroy
their review decisions, hand-written games, upload log, and photos.

Two layers. The README's "Bring your own shelves" now leads with
`uv tool install git+…` and running in a directory of your own: data
lands untracked by construction and a bug fix is `uv tool upgrade`,
which cannot touch it. And because nobody re-reads a README, Config
gains tracked_data_warning(): if artifacts under data_dir are
git-tracked, `bggpipe init` and the web dashboard both warn in plain
words. The owner's exemption is data/.own_repo — a GITIGNORED marker,
so the author's checkout is silent while a fresh clone of the same
repo still gets the warning (a committed marker or config key would
have shipped the exemption to exactly the people who need warning).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jXZFSTZQKzAC8fqpWSz9g

tests/test_config.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import os
 from pathlib import Path
 
 import pytest
@@ -96,3 +97,44 @@ model = "claude-sonnet-5"
         pytest.warns(UserWarning, match="anthorpic"),
     ):
         load_config(p)
+
+
+def _git(tmp_path, *args):
+    import subprocess
+
+    subprocess.run(
+        ["git", *args],
+        cwd=tmp_path,
+        check=True,
+        capture_output=True,
+        env={
+            "PATH": os.environ["PATH"],
+            "HOME": str(tmp_path),
+            "GIT_AUTHOR_NAME": "t",
+            "GIT_AUTHOR_EMAIL": "t@t",
+            "GIT_COMMITTER_NAME": "t",
+            "GIT_COMMITTER_EMAIL": "t@t",
+        },
+    )
+
+
+def test_tracked_data_warning_fires_only_inside_a_repo(tmp_path):
+    """The clone-and-run trap: data at git-tracked paths is one panicked
+    `git reset --hard` from destruction. Outside a repo, silence."""
+    data = tmp_path / "data"
+    data.mkdir()
+    (data / "matches.csv").write_text("x")
+    cfg = Config(data_dir=data)
+    assert cfg.tracked_data_warning() is None  # no repo, no trap
+
+    _git(tmp_path, "init")
+    assert cfg.tracked_data_warning() is None  # repo, but nothing tracked
+    _git(tmp_path, "add", "data/matches.csv")
+    _git(tmp_path, "commit", "-m", "seed")
+    warning = cfg.tracked_data_warning()
+    assert warning and "git-TRACKED" in warning and ".own_repo" in warning
+
+    # the owner's exemption is a gitignored marker: a fresh clone of this
+    # repo would NOT carry it, so only the author's checkout is silenced
+    (data / ".own_repo").touch()
+    assert cfg.tracked_data_warning() is None