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

.gitignore

@@ -2,6 +2,7 @@
 .env
 *.storage_state.json
 data/.lan_key
+data/.own_repo
 playwright/.auth/
 storage_state.json