Prepare source repository for GitHub with asset exclusions and README

docs/repository-content.md

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+# Repository content
+
+Private runtime assets, extracted maps, custom animation clips, build output,
+settings and logs stay local. They are excluded by `.gitignore`.
+
+The two project-owned binary fixtures are generated without game assets:
+
+- `maps/format-demo.skate`: `python tools/make_skate_demo.py`
+- `crates/skate-data/tests/fixtures/retail-collision.rwcmset`:
+  `python tools/make_collision_fixture.py`
+
+Run these commands from the repository root. Both generators reproduced the
+tracked fixtures byte for byte during publication preparation.
+
+The README artwork in `docs/images/skating-crab.png` was supplied by the project
+owner for publication. Other binary resources belong to the vendored Bevy dependencies, whose license
+files remain alongside their source. Do not add extracted game content to
+these directories or force-add ignored private files.
+
+The pre-publication audit inspected all reachable historical blobs for private
+paths, matches against local private-file hashes, unexpected binary files and
+common credential patterns. No matches requiring removal were found.