library-rag-export.service now regenerates reading_umap.json on cyrion
whenever the corpus or its located highlights move, writing straight into this
checkout. Keeping it tracked would defeat that: a generated file that changes
under git dirties the tree, which both flips auto-build.sh's change signature
and breaks the `git pull` at the top of every build tick — the reason
notebooks-export/ is already ignored, in that file's own words.
So it joins them: ignored, plus a find line in the signature block so the
5-minute tick still notices a new projection.
The cost is that a fresh checkout has no map until the service runs — on
cyrion that is within the hour, anywhere else never. ReadingMap.astro says so
where someone would hit it.