feat(admin-config-tool): task 1.3 - nix and xce-build.sh integration

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

src/xceadmin/.gitignore

@@ -0,0 +1,12 @@
+# Build artifacts
+build/
+*.egg-info/
+__pycache__/
+*.pyc
+.pytest_cache/
+.venv/
+.ruff_cache/
+
+# Synced package data — regenerated from src/schema/ and src/configs/schema/
+# by `xce-build.sh xceadmin`'s schema-sync step; never hand-edited, never committed.
+src/xcecommon/schema_files/

src/xceadmin/pyproject.toml

@@ -27,3 +27,12 @@ include = ["xceadmin*", "xcecommon*"]
 
 [tool.setuptools.package-dir]
 "" = "src"
+
+# Bundled copies of controller.capnp/c.capnp and the 7 JSON validation
+# schemas (synced from src/schema/ and src/configs/schema/ by
+# src/tools/xce-build.sh's `xceadmin)` case, see Requirement 5.5). Without
+# this, a non-editable `pip install .` builds a wheel containing only *.py
+# files and silently drops schema_files/ — an editable install would never
+# catch this since it resolves straight into the checked-out source tree.
+[tool.setuptools.package-data]
+xcecommon = ["schema_files/*.capnp", "schema_files/json/*.schema.json"]