Add python3 + pycapnp/pyyaml/jsonschema/click/pytest/ruff (+pip) to the
nix dev shell, wire a new `xceadmin` case into xce-build.sh (schema
sync from src/schema/ and src/configs/schema/, pip install by profile,
pytest), package-data config so bundled schemas survive a non-editable
install, xce-build SKILL.md docs, and ruff format/check hooks matching
this repo's stdin-JSON hook convention (not brief.md's env-var sketch,
which would have silently no-op'd).
Verified live end-to-end inside `nix develop ./src` for both the
default (editable) and release (pinned) profiles; found and fixed 3
real bugs along the way (missing pip, PEP 668 externally-managed-
environment, missing package-data for release-profile bundling) —
logged as bug-1458/1459/1460.
Requirements: 5.5, 8.1, 8.2, 8.6
